Harry finds evidence that convinces him that all his friends have betrayed him.
He wreaks horrible bloody (literally) and undoable revenge.
He finds a final piece of evidence that proves he was mistaken and/or
One take: make the 'evidence' be a really badly thought out prank by sirius, remus, the twins, or all of them together. Or maybe they each seperately play different pranks on harry which lead him to his belief in betrayal.