Obviously there's a lot we don't know, which makes me hesitant to guess. Stuff might come out in the trial that takes things one direction or another.
From what we do know so far, it seems like a range of scenarios are possible. For example:
Zimmerman is doing his neighborhood watch thing and sees Trayvon walking along looking at houses (trying to figure out which one is his dad's girlfriend's house, they're hard to tell apart). Zimmerman is reminded of the robberies in the area and decides to keep a close eye on this guy. Zimmerman's feeling guilty that the robberies happened on his watch and is determined to not let it happen again.
Zimmerman says something to Trayvon, they have a brief exchange in which Zimmerman becomes more convinced that Trayvon's up to no good and Trayvon becomes more convinced that this guy is creepy, and Trayvon takes off and Zimmerman goes after him.
There is some sort of physical confrontation and Zimmerman feels threatened, and shoots Trayvon.
Not sure what charge that would be, if that's the scenario. Seems like he'd still have to be guilty of something since all of that chasing is contradictory to Stand Your Ground.
Zimmerman is angry about something maybe, generally loaded for bear. Sees Trayvon doing nothing in particular and gets pissed off. Goes after him with the intent of causing harm. Trayvon does nothing but try to defend himself when physically confronted, and Zimmerman still shoots him.
That would seem to get more towards actual murder.
I think manslaughter is the more likely conviction.
But there really is a lot we don't know.