It has also been suggested that the phrase may have its origins in the infamous Jonestown murder-suicide which occurred in 1978. During this horrific event, hundreds of people in a cult called the People's Temple died by drinking poison-laced drinks or being forced to consume poison. According to popular mythology, the poison was mixed with Kool-Aid. In fact, as ample evidence from the period indicates, the Jonestown suicides actually drank Flavor Aid, a slightly different drink.While the brand name may have been Flavor Aid vs Kool-Aid, that distinction is rather meaningless. Kool-Aid has become such a dominant fixture in the "flavored drink" market, that the brand name has become the generic term for the product (like BandAid for adhesive bandages, or Kleenex for facial tissues). So people refer to a flavored punch drink as Kool-Aid when it might be Flavor Aid or the generic store brand drink.
As for the origin of the phrase "don't drink the kool-aid" or its variations, this phrase was not in common usage in the 60's and 70's and the whole acid-laced Merry Pranksters phenomenon was isolated enough to a fringe group in society that it would have the broad exposure and context for the phrase to be meaningful in the overall lexicon. It is pretty obvious that the phrase in broad usage originated after the Jonestown massacre, referring to the forced poisoning of the cult followers.
Nicely done.