Alright so, Snape and Dumbledore had been having a very secret love affair for about since Lily bitch-slapped Snape in his incredibly large nose. Anyhoo, so the Potter kid is like hiding under his Invisiblity Cloak, and totally breaks down laughing and whistling at Snape, because the Potions master doesn't want to kill Albus Percival Wulfric Brian Dumbldore. So Snape says Avada Kadava, begins to turn to Potter, hits Dumbledore, and 'accidentally' pokes Belatrix in her eye with his wand. So then of course, she starts throwing around Killing Curses, Malfoy almost falls out of the window Dumbledore had not thirty seconds before, and just about everyone in the room gets mad at one another. By the time bricks were falling out of the walls just to get away from the hundred and one curses being casted (and dodged), Snape had grabbed Malfoy by the collar, located Potter, and attempted to escape down the stairs. That's when Belatrix's husband-man notices them and aims his wand at Malfoy, thinking the kid with black hair and an angry look belongs to the infamous Black family (or some such nonsense), and thus is nothing to worry about (I mean, come on, he's probably in Sytherin and he wasn't attacking the Death Eaters). So Snape gets pissed, turns to Potter and insists, "Potter give Malfoy your Cloak! He's a complete twat, then we can get the hell out of here!"
"Why should I do what you say, why should I help him, holy shit, is that Bill downstairs yelling something about getting bitten by a werewolf, and why in the name of all that is not Voldemort related is anyone else from the Ministry or Order not here yet?"
"Give me the damn Cloak!" Snape proceeded to snatch it right out of Potter-Wotter's hands. He threw it over Malfoy's head, gave Husband the finger, and shoved both boys down the stairs.
Then since Bill was lying on the floor trying to recover from Greyback biting his freaking arm off, Malfoy tripped over him, Invisibiltiy Cloak falling, and bashed his head quite soundly into the nearest wall, where various black spiders took up residence inside his hair (he would be finding them still there in three months to come). Snape ran out the door with Potter running after him screaming about Horcruxes and Slughorn's inability to be normal and crazy fire and zombies.
Because Belatrix was apparently there and she was the one who released the Dark Mark into the sky, she suddenly gets with a Stunning Spell on her way down the stairs, and the guy that was supposed to do the Dark Mark came down right behind her, only to get hit by the Killing Curse like he was supposed to. It's safe to assume Crazy Lady runs down the hall as well, laughing maniacally at her fallen comrade.
"Yep, I totally did that on purpose! Hey Greyback, wanna have a love affair with me in this random broom closet…no? Busy eating that kid? We'll have a rain check then!" With a nod, she runs outside to set fire to a certain hut. She sees Snape, says some particularly naughty words, and the giant squid reaches out and starts thrashing her around. At which point, Harry is screaming something about how only the half-blood prince can save them now and how Snape can only make matters worse, but before the boy can even blink, Snape backhands him.
"I'm the Half-Blood Prince, you jackass! (omg! finally I get a book named after me!)"
Potter has a heart attack and dies on the spot, on which Fang decides he has to go poopie and stuff.
Belatrix, Hagrid, and Fang slap their hands against their foreheads, so does the wounded McGonnagal, so do all the Death Eaters, so do all the Hogwarts children pretending to sleep, so do all the Order members, so does Hermione (Ron doesn't get it), so does the still-knocked-out Malfoy, so do the spiders, so does every Harry Potter fan, and last, but not least, Dumbledore does too(even though his muscles probably only just spasmed, since he's dead and all).
And everyone not mentally or physically dead or scarred lived happily ever after (the giant squid and the oblivious Fudge).