Slytherin hate Gryffindor and are far more ruthless (morals be damned, ruthlessness matters). They blitz their hated enemy and Gryffindor, being all brave and stubborn, don't back down. Pyrrhic victory for Gryffindor.
Hufflepuff, having no one that matters except Diggory, get eaten alive by Durmstrang, who proceed to whoop Beauxbatons. Pyrrhic double victory.
Ravenclaw being smarter than everyone wait it all out. Gryffindor and Durmstrang team up on them. They eventually overcome them, and only Harry, Ron, Hermione, Neville, and Krum are left standing. But at the last minute Krum stabs them all in the back. He bombarda's the lot of them and Avada's them before they recover. But Neville recovers in time to disarm him before it's his turn and subsequently kills Krum.
The Ultimate Showdown of Hogwarts Destiny ends with Neville Longbottom in a blood stained sweater.