Why does Kool-Aid relate to being something's fanboy/fangirl?
I think you are referring to the metaphor of drinking the koolaid:
Wikipedia says that the origins are:
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:
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:
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.