Can't believe they actually went with that ending. *SPOILERS*
First off, it directly contradicted an earlier plot point, what with the ring and all.
Second off, Brian suddenly started acting like an idiot when the handwave for said plot point requires him to be an absolute genius and have at least one other person in on it.
Third off, there wasn't any actual payoff for the hunter joke or the brakes scene. Seriously, what? They missed an opportunity to have extra gore and give the somewhat-sympathetic killer a bittersweet ending (he dies, but so do his targets).
Fourth off, it just strikes me as an asspull. In fact, in the copy machine scene, the guy killing the vice principal is far too muscular to be Brian. In fact, he looked a fair bit like the custodian; seeing as how the custodian is also nuts, he was the less obvious but still reasonable choice for the murderer.
That ending was LITERALLY FOUR KINDS OF BULLSHIAT.