He might have seemed an idiot/easy target for conversion in his pink T shirt and pink pussy-cat hat. Perhaps ISIS sent him on a drug-induced rampage with promises of paradise? Drugs are the powerhouse that fuel the middle east atrocities...drugs are a thousand-year old tradition in islam. Search the 'Old Man of the Mountain' which gave rise to the term Assassin.
Long shot? You betcha. But that fellow Holmes who shot up the movie theatre with his hair died bright henna-red (in the tradition of mohammad himself) maintained he didn't know how he got there, or why he did it.
Scopolamine is an hallucinogen mind control drug for example. Comes from the datura plant. Can't find a motive? Maybe some fanatical muslims infiltrated the anti-Trump protests in which he was active, and used him as a patsy for the glory of allah who hates gambling and alcohol.
Now there is a motive.
Jihadis have used drugs for a long, long time. Assassins - from doping up on hashish before action.
My good friend in the Philippines whose son is in the PI special forces fighting jihadis in Mindanao said the PI military found huge factory and tons of meth (some other name she called it) that they sell all over the PI to make money, plus all the fighters there are totally doped up on it.