Thank you so much for taking the time to write this. I had postulated a bit more crime in the killer's background, but I count dealing in drugs (probably in a fairly minor way) to be at least on a continuum of breaking rules and laws.
My top scenario is indeed "former student," someone who is 3-5 years older than the undergrads, didn't finish, is still living on campus somewhere (this happens at most colleges, various ways to do it), and yes, has found a way of staying relevant to students by always having blunts to sell or something like that. I think he's mostly a loner and enjoys observing (and perhaps mocking) others, rather than building relationships.
Your description of what he got out of these killings is chilling. It very much resembles what others are sharing about some of the more random serial killer types (or one-off killer types).
Lots of food for thought.