Started getting good when...[SPOILERS]
...Leon got killed.
Actually, I started to get into before you know for sure what happened with the stand-off among Leon, Mac and Lise. You know a gun went off, but you don't know who got shot...And you know that Leon is cuckoo-bananas, so for a little while you don't really know for sure who is messing with the remaining campers.
But really, once Leon was out of the picture, I started to sympathize much more with all of the characters, and was less annoyed. I mean, he was already set up as a typical alpha-male douche, but then in the night scene on the bridge he goes on what seems like a kind of drug-induced freakout, apologizing for acting like an ass one second, then practically the next second almost dropping Gray off of the bridge and then going into a rage and attempting to rape his girlfriend? (Seriously, did I miss a quick part in there where he does a bunch of coke?)