I disagree. Harry watches Quirrel stun the professor and 3 Aurors, let them tumble off their brooms and false memory charm them, but Quirrel hasn't done anything wrong? That's, what, 8 felony equivalents at least? (Assault x4, and presuming False Memory Charm would be at least equivalent to assault, probably more like rape).

Filch's crime, for which Harry wants him to serve jail time, is that he sent Draco & co to the Forbidden Forest, potentially exposing them to assault. Quirrel actually assaulted them.

To grossly simplify, there's a consistent set of ethics that says that Filch and Quirrel both need to be punished. (action -> consequences) There's another, which says both ought to be forgiven. (no harm, no foul) Forgiving those who are cool and punishing those who are lame is unethical, particularly given the disparity between their offenses.

On another tack, how on earth can Harry know anything about the Centaur? It attacked a child after rambling on for a while. Jumping to the conclusion that its fixated on him and won't just attack some other child is really arbitrary. I mean, plenty of serial bad guys fixated on their victims, it doesn't make them safe for other folks. Today 'the stars' told Centaur to kill Harry. Tomorrow they tell him to kill Ron, or Hagrid, or set himself on fire. I wonder what they told him to do yesterday?