adult trip

In the book, he is constantly smoking weed, even as a teen, so there is that (I don't remember if this aspect was emphasized or mentioned in the movie at all)

There is certainly some kind of karmic justice in that, if you will, but obviously his problem and the subsequent rehab was not the direct consequence of him mistreating Lux.

"He didn't strike me as the type to end up that way."

You can never fully know what goes on in people's heads. You can never truly know them, not even in real life, much less can you deduce the personality of a fictional character from a few brief scenes. Such is life. You know in TV when a serial killer is revealed to be one and the TV-crew interviews the neighbors? They usually say "He was always so nice and polite, I really had no idea he is capable of something like this." - well, it's quite like that in the case of Trip. He was cool for sure, and seemed mentally more stabile than most of the other boys we see in the film, but his coolness really was just a facade to hide his emotional instability.

I recommend you the book, there is some background info on the character and all of the events of the moview, which helps putting everything into perspective.

"I was looking back to see if you were looking back at me to see me looking back at you"