Why did he... (SPOILERS)
First of all: I liked this movie alot. They did a really good job of making all the characters likeable, which doesn't happen very often in simple horror movies.
But there was one thing I didn't quite get: Why did Clutch jump off the building in the end? Was he bitten? Because he touched his neck and looked at his hand right before he jumped. And also his friend, who has become a rat-man by then, didn't attack him. But then again, I didn't see him getting bit.