And he could have done worse than he did. Trevor's arm might be broken, but otherwise he's fine.

[Which brings him to:] Trevor is a vampire hunter. It's what his family does--it's a sacred calling for them. He was aware at a certain point that the Enclosure wasn't real, or at the least he knew no one was burning alive. But Sephiroth resembled the kinds of vampires he's fought before. That's why he attacked.