Kool-Aid is a reference to the poison kool-aid that infamous cult leader Jim Jones had his followers drink en-masse in the Jones town massacre in Guyana in the 1970's. Younger people would probably not immediately get this reference if they were born after 1970 or too young to remember it in the news. Hundreds of men, women and children were killed.
Jim Jones started out as a regular evangelical preacher, who was very charismatic and develped quite a following after himself. He started to try and assert more and more control over his church and eventually started an isolated commune of sorts called "Jonestown" in the jungle of Guyana. He became more and more egomaniacal and paranoid and many allegations of abuse were leveled at him by the families of some of his followers. They claimed they were being held by force and not allowed to leave. Some journalist and a senator I believe came to Jonestown to check it out and as they were about to board the plane to fly back to the states, Jim Jones had them shot. He knew then it would only be a matter of time before the authorities came for him, so he put into effect a mass suicide plan for the commune. They served the children the poison kool-aid first, then the parents, and so on up the heirarachy. Some people tried to flee into the jungle and were shot. (A few might have escaped, I can't remember). When the authorities did come they just found a mass of dead bodies. There have been a few movies made about it. You can check the video store archives if you are interested. It is a fascinating story.
The analogy is used to compare the watchtower doctrine as similiar death-dealing "poison". This analogy may seem a stretch to some, but in reality, witnesses are willing to sacrifice their very lives and their children's lives to follow the society doctrine on blood transfusions even though they make no medical sense and no other relgion in the world views the bible's dietary sanctions on blood as applying to life saving blood transfusions. By signing the no blood cards, witnesses are agreeing in advance, in writing, to "drink the kool-aid" that the society gives them, in any flavor or color, they choose to serve it.
Hope this helps
Cog