While use of the phrase dates back to 1968 with the nonfiction book The Electric Kool-Aid Acid Test,[1] it is strongly associated with the events in Jonestown, Guyana, on November 18, 1978, in which over 900 members of the Peoples Temple movement died. The movement's leader, Jim Jones, called a mass meeting at the Jonestown pavilion after the murder of U.S. Congressman Leo Ryan and others in nearby Port Kaituma. Jones proposed "revolutionary suicide" by way of ingesting a powdered drink mix made from Flavor Aid, later misidentified as Kool-Aid, that was lethally laced with cyanide and other drugs.[2][3]

Via https://en.wikipedia.org/wiki/Drinking_the_Kool-Aid