Why does Kool-Aid relate to being something's fanboy/fangirl?
I think you are referring to the metaphor of drinking the koolaid:
"Drinking the Kool-Aid" is a metaphor, used in the United States and Canada, that means to become an unquestioning believer in some ideology, or to accept an argument or philosophy wholeheartedly or blindly without critical examination. The phrase can sometimes have a negative connotation, or can be used ironically.
Wikipedia says that the origins are:
The basis of the term is a reference to the November 1978 Jonestown Massacre, where members of the Peoples Temple were said to have committed suicide by drinking a "Kool-Aid"-like drink laced with cyanide.
Wikipedia is not the only source that links the origins of the phrase to the Jonestown Massacre. This article by the Center for European Reform, this page by the San Diego State University and this page by MSNBC all support the findings from Wikipedia.
Also, on the alternate possibility that the phrase comes from the so called "Acid Tests", Wikipedia notes:
The expression has also been used to refer to the activities of the Merry Pranksters, a group of people associated with novelist Ken Kesey who, in the early 1960s, traveled around the United States and held events called "Acid Tests", where LSD-laced Kool-Aid was passed out to the public (LSD wasn't deemed illegal in the U.S. until 1966). Those who drank the "Kool-Aid" passed the "Acid Test". "Drinking the Kool-Aid" in that context meant taking LSD. These events were described in Tom Wolfe's 1968 classic The Electric Kool-Aid Acid Test. However, the expression is never used figuratively in the book, only literally.
So, while kool-aid seems to have taken on the drug association in the late 1960s, it may not have been used as a figurative expression until the 1970s--after the Jonestown Massacre.
Note also that MSNBC found that while Jonestown is connected to the origins of the metaphor, the original drink was not Kool-Aid:
One little known footnote: the fruit drink actually used at Jonestown on that day was a British product, Fla-Vor-Aid. In Guyana, it was cheaper than Kool-Aid.
I think the answer is more likely related to the "Electric Kool-aid Acid Test", referring to Koolaid containing LSD, rather than Jonestown. One hears things like "What kind of Kool-aid has he been drinking" and similar expressions when someone is acting or speaking irrationally. This pre-dates Jonestown by over a decade, and I personally heard it used in that era. Or at least, I believe I did, but maybe I've just lost a few brains cells...;)
As stated repeatedly, Wikipedia is NOT the gospel.