Just a psychopath and violent criminal.
Unfortunately I think the alternative/activist groups are easily infiltrated by these types, who can be quite charismatic at whipping up people to act as mobs and follow their crusades.
As vacuous as some celebrities are, I can’t find any justification whatsoever for their violent murder, all this sort of thing does is gives more ammunition to the anti-drugs lobby and the puritan types who use this as an “I told you so” argument against alternative culture.