Of American-English origin, the phrase to drink the Kool-Aid, and its variants, have two acceptations:
– to commit suicide;
– to demonstrate unquestioning obedience or loyalty.

For example, the following is from Authority of Boris Johnson damaged as own MPs say ‘gig’s up’, by Jessica Elgot and Heather Stewart, published in The Guardian (London and Manchester, England) of Thursday 21^st April 2022:

Tobias Ellwood, the Tory chair of the Commons defence committee, who has already called for the prime minister to go, tweeted that it was time for his colleagues “to stop drinking the Kool Aid”.

The phrase to drink the Kool-Aid refers:
– to Kool-Aid, a proprietary name for a powdered concentrate which is added to water or another liquid to make a fruit-flavoured drink;
– to a mass suicide, on Saturday 18^th November 1978, by members of the Peoples’ Temple, a religious and political movement, in Jonestown, Guyana, who drank a cyanide-laced drink thought to be similar to Kool-Aid—and, in the sense to demonstrate unquestioning obedience or loyalty, to the obedience those members demonstrated towards their leader, Jim Jones (1931-1978).

The following is from the Journal and Courier (Lafayette, Indiana) of Tuesday 21^st November 1978:

GEORGETOWN, Guyana (AP)—U.S. troops flew in today to begin a macabre shuttle ferrying out corpses of more than 400 American cultists who drank a lethal brew of Kool-Aid and cyanide in fanatic loyalty to a suicidal messiah. […]
[…]
THE BODIES OF sect founder the Rev. Jim Jones and his wife were among 409 corpses that a police spokesman said Guyanese troops had counted in and around the meeting hall in Jonestown. Jones and several others had been shot, presumably by their own hand. The others had drunk Kool-Aid into which the camp doctor mixed cyanide.
Jones ordered the mass suicide Saturday after sect members ambushed and killed a U.S. congressman and four other persons who were part of an investigative team that visited Jonestown.

It seems that it may have not been Kool-Aid that was used in the mass suicide—as mentioned in the following from The Sacramento Bee (Sacramento, California) of Thursday 7^th December 1978:

Fatal Potion May Not Have Had Kool-Aid Base
By PAUL CLEGG
Bee Staff Writer

Kool-Aid, cyanide and mass suicides in Guyana? Who would have thought it? Kool-Aid is the drink of childhood, the “bug juice” of a thousand summer camps, the incipient capitalism of city street corners, 10 cents a glass.
In reality, Kool-Aid may not have been the sweetened coating on the poison consumed by many of the 900 members of the Peoples Temple. The product may simply be paying the price of being No. 1 in the powdered soft drink field. Think of such products and Kool-Aid pops into mind.
Initial accounts from survivors of the Jonestown tragedy told of a fatal potion of purple Kool-Aid, tranquilizers and cyanide. But later, a reporter on the scene wrote that he found hundreds of packages of Fla-Vor-Aid in an open carton just 30 yards from where the cyanide mixture was distributed. Fla-Vor-Aid, a product of Jel-Sert Inc. of Chicago, is similar to Kool-Aid.

The earliest occurrences of the phrase to drink the Kool-Aid and variants that I have found are as follows, in chronological order:

1-: From Bench performances egged teams on in Egg Bowl, the account of a football match between the Mississippi State and the Ole Miss Rebels, by Don Bowman, Clarion-Ledger sports writer, published in The Clarion-Ledger (Jackson, Mississippi) of Sunday 26^th November 1978:

“Hey,” yelled an Ole Miss offensive lineman after a Marler pass was dropped by one of his backs, “I’ve seen better hands on a clock.”
“You guys play offense?” asked another. “If I were one of your fans, I’d drink some Kool-Aid.”

2-: From Nelson’s Only Human, Admits Mistakes, an interview of Donald Nelson (born 1940), head coach of the Milwaukee Bucks, by Jill Lieber, sports columnist, published in the Milwaukee Sentinel (Milwaukee, Wisconsin) of Friday 1^st December 1978:

“You know, basketball is such a part of me. It has become a way of life. All the injuries we’ve had this year haven’t depressed me. As a coach I find them a challenge. The losing doesn’t get me down like it once did. I am used to the traveling and being away from home. Sitting on a plane is like sitting in my living room.
“And when it all stops, when it’s all over? That’s when you drink the Kool Aid.”

3-: From ‘There Ain’t No Santa Claus’, by Charles King Jr., who “conducts “sensitivity sessions” for black-white understanding”, published in The Atlanta Constitution (Atlanta, Georgia) of Friday 22^nd December 1978:

My child, you must keep your personage inextricably wrapped around the world of reality. Let your joys explode around real things, real people. Strongly resist overpowering personalities that impact upon your weaknesses and frailties . . . let your naivete vanish early in life, and the bud of your innocence flower into maturity. Adults who remain children drink cyanide in Kool-Aid, destroying their own lives because the will-of-the-wisp cannot be captured or contained. We are ashamed to will you such an untidy and uncomfortable existence, but so it must be.

4-: From Cleveland jokes convey some truth: Many people don’t take part any more, by C. Fraser Smith, Sun staff correspondent, published in The Sun (Baltimore, Maryland) of Friday 22^nd December 1978:

To dramatize his charge that one Cleveland bank is responsible for the default, the mayor went to Cleveland Trust Monday and closed his savings and checking accounts, which contained about $9,000.
Not everybody in Cleveland was impressed.
“Here’s Dennis,” says Jack E. Peters, an engineer with the Eaton Company, “taking his piddling $9,000 out of the bank and saying, ‘We don’t like you; we’re not going to play any more.’”
To Mr. Peters, who like many native Clevelanders now lives in the suburbs, the $9,000 may have been “piddling,” but the mayor was banking on his view that many who still live in the city would understand and copy his gesture.
If they did, it would be no surprise to Councilman John J. Lynch.
“To the people who support Dennis Kucinich, Dennis Kucinich could say, ‘Drink the purple Kool-Aid,’ and they would,” he said.

5-: From Garner Ted says he saw storm coming, an Associated-Press report published in the Corpus Christi Times (Corpus Christi, Texas) of Wednesday 10^th January 1979:

TYLER (AP)—Garner Ted Armstrong, the onetime heir to the embattled Worldwide Church of God said he’s neither pleased nor surprised by the dark clouds swirling around his father’s California-based religious empire.
[…]
He said he could see a tragedy in the Worldwide Church of God along the order of the mass suicide in Guyana.
“When the news came from Jonestown, I lost four nights of sleep, praying for peace of mind,” Armstrong said. “One woman told my sister in California that if Herbert W. Armstrong told her to drink that Kool-Aid she would do it. That frightens beyond words.”

6-: From When Churches Fail, Some Turn to Cults, by George R. Plagenz, Scripps-Howard religion writer, published in the Memphis Press-Scimitar (Memphis, Tennessee) of Saturday 13^th January 1979:

Guyana is the story of what can happen in a society where people’s expectations are raised too high, where they are told that “yours is the world and everything that’s in it.” ¹
Sooner or later the realization hits home that they have been lied to and betrayed. The promises are not about to come true.
Some in desperation take refuge wherever it is offered—and end up drinking the Kool-Aid flavored hemlock.

¹ This refers to the line “Yours is the Earth and everything that’s in it” in If, a poem by the British novelist, short-story writer and poet Rudyard Kipling (1865-1936).

7-: From a letter by one Gerald D. Cranford, published in the Capital Journal (Salem, Oregon) of Friday 16^th February 1979:

Mr. Herbert W. Armstrong and the Worldwide Church of God have been somewhat in the news the past few weeks. As a result, some may think that we, the members of the Worldwide Church of God, are a bunch of dimwits following a “senile” leader who has put together a weird collection of beliefs and teachings.
There was a comment made a short time ago that if he (that “senile” leader) said, “Drink Kool-Aid,” his followers would.

8-: From an interview of the U.S. stuntman Evel Knievel (1938-2007) by Mike Duffy, Free Press staff writer, published in the Detroit Free Press (Detroit, Michigan) of Friday 18^th May 1979—the phrase is to wait in line for a drink of Kool-Aid:

Q—Did your prison experience change you at all?
A—Oh, it made me aware of what a sad state this United States of America is in. I feel that we’re . . . being sucked into communism because of the socialistic leaders that we’ve had and because of the promises that have been made to the people. Our government kind of reminds me of Jimmy Jones. I think we’re all just waiting in line for a drink of Kool-Aid. It’s a sad situation.

9-: From a correspondence from Washington, D.C., by Robert S. Boyd, Herald Washington bureau chief, published in The Miami Herald (Miami, Florida) of Saturday 21^st July 1979:

“Don’t drink the Kool-Aid,” was the gag of the day in the White House press room when the mass offers to resign were announced.

10-: From Face It, The Chief Bombed, a correspondence from Washington, D.C., published in The Daily Oklahoman (Oklahoma City, Oklahoma) of Sunday 22^nd July 1979:

President Carter ² fired some of his top helpers, apparently because he thought they were disloyal.
[…]
Nobody in the White House or Edward’s office is drinking Kool-Aid these days.

² The Democratic statesman Jimmy Carter (born 1924) was the 39^th President of the USA from 1977 to 1981.

11-: From The Tuesday Line, by Terry Jones, published in the Edmonton Journal (Edmonton, Alberta) of Tuesday 7^th August 1979—the Calgary Stampeders are a Canadian football team based in Calgary, Alberta, and the B.C. Lions are a Canadian football team based in Vancouver, British Columbia:

Calgary Stampeders came into Edmonton last week cocky and convinced they were the new power in the West.
Being that the final score was 44-9 for Edmonton, and Calgary fans went home and considered drinking grape Kool-Aid, it would be reasonable to expect a somewhat different approach from the Lions.
They’ve done just about everything but change their came to the B.C. Lambs for this one.

12-: From a letter by one Robert J. Cornell, published in The Courier-News (Bridgewater, New Jersey) of Thursday 6^th December 1979:

Many think the Watchung Republicans are no longer a party but have become a “cult.” This smug, unresponsive cult has asked the faithful to do just about everything except drink Kool-Aid.

13-: From Most Connally staff now ‘volunteers’, about John Connally (1917-1993), who sought the Republican nomination for President of the USA, by Ford Fessenden, News staff writer, published in The Greenville News (Greenville, South Carolina) of Wednesday 20^th February 1980—Gay Suber was acting as chairman for John Connally’s campaign:

Suber said Reagan ³ draws on a core of committed supporters that won’t grow.
“Reagan has a hard-core base of support, almost a cult following,” he said. He likened Reagan backers to followers of the Rev. Jim Jones, who committed mass suicide in a village in Guyana. “They’d drink the Kool-Aid if he asked them,” Suber said.

³ The Republican statesman Ronald Reagan (1911-2004) was the 40^th President of the USA from 1981 to 1989.

14-: From Another stock guru speaks, by Richard Stern, published in the Daily News (New York City, New York) of Thursday 8^th January 1981:

Joe Granville […] sent the market into its incredible slide yesterday […]. One of our astute contacts yesterday called Granville “the Jim Jones of the stock market, telling his devoted disciples, the investors, to drink Kool-Aid.”

15-: From Jerry Falwell: Lynchburg loves, hates TV evangelist, about the U.S. televangelist Jerry Falwell (1933-2007), by Tracie Rozhon, of the Independent News Alliance, published in The Minneapolis Star (Minneapolis, Minnesota) of Monday 16^th February 1981:

To Ann Dill, whose husband recently retired from an executive job at the Continental Can Co., Falwell’s influence on his followers is a bit frightening.
“You know, when that Jonestown thing happened, the Jim Jones thing, I heard a lot of people around here saying that if Jerry Falwell told his people to drink [poisoned] Grape Kool-Aid, they’d drink Grape Kool-Aid.”

16-: From Ginsberg Tells Of Kool Aid Of Nuclear Power, the account of a speech that the U.S. poet Allen Ginsberg (1926-1997) delivered at Gettysburg College, by M. Eileen Graham, Times correspondent, published in The Gettysburg Times (Gettysburg, Pennsylvania) of Friday 20^th February 1981:

“We are all being put in the place of the citizens of Jonestown, being told by our leaders to drink the Kool Aid of nuclear power,” he said stressing his dismay at the return to right-wing politics and morality in America.