Anna is maybe 3/4ths of the way through the book so SHHH NO SPOILERS
Because he's always lurking by the girls' bathrooms. Really, like, all the dark corners. And he spent the whole summer up in his room. And he's so grumpy.
Maybe thought he could handle the monster because he's always hanging out with monsters in his books. Then, the monster put a curse on him, and now he's hypnotized to be on the monster's side.
Because he's evil and he probably framed Hagrid for opening the chamber fifty years ago.
He also just has that evil tone about him.
Update: The Reveal