Setup:
'He'd have me!' said Bellatrix passionately. 'I, who spent many years in Azkaban for him!'
'Yes, indeed, most admirable,' said Snape in a bored voice. 'Of course, you weren't a lot of use to him in prison, but the gesture was undoubtedly fine – '
'Gesture!' she shrieked; in her fury she looked slightly mad. 'While I endured the Dementors, you remained at Hogwarts, comfortably playing Dumbledore's pet!'
'Not quite,' said Snape calmly. 'He wouldn't give me the Defence Against the Dart Arts job, you know. Seemed to think it might, ah, bring about a relapse … tempt me into my old ways.'
'This was your sacrifice for the Dark Lord, not to teach your favourite subject?' she jeered. 'Why did you stay there all that time, Snape? Still spying on Dumbledore for a master you believed dead?'
'Hardly,' said Snape, 'although the Dark Lord is pleased that I never deserted my post: I had sixteen years of information on Dumbledore to give him when he returned, a rather more useful welcome-back present than endless reminiscences of how unpleasant Azkaban is …'
'But you stayed – '
'Yes, Bellatrix, I stayed,' said Snape, betraying a hint of impatience for the first time. 'I had a comfortable job that I preferred to a stint in Azkaban. They were rounding up the Death Eaters, you know. Dumbledore's protection kept me out of jail, it was most convenient and I used it. I repeat: the Dark Lord does not complain that I stayed, so I do not see why you do.'
"Coward." said Bellatrix
"WHAT did you just call me" snarled Snape
"DON'T YOU DARE TALK BACK TO ME, YOU FILTHY HALF-BLOOD"!
"CRUCIO"! screamed Bellatrix.
"Sectumsempra"! Snape fired back. Peter scrambled under the couch, shaking uncontrollably.
"Bella, Bella STOP"! Screamed Narcissa.
"Avada Kedavra"! Bellatrix shot in return. Narcissa ducked to avoid the blasting ray of green light, which Snape promptly deflected, before responding with a consecutive flurry of curses.
Soon, Narcissa and Bellatrix simaultaneously were firing at Snape, who was now defending from two sides.
"Cissy, stay out of this"! "I can handle the Half-Blood myself!" shouted Bellatrix, but it was too late. The two witches had already converged on Snape, who was now furiously spitting curses out of his wand in a rapidfire motion. Jets of light began to illuminate the room as spells flew constantly. "Incendio"! Bellatrix commanded. A feral serpent of flame slammed into the wall, setting it ablaze.
"Avada Kedavra"! Yet another Killing Curse emerged from Snape's wand. This time, it struck. Bellatrix cringed momentarily, before crumpling lifeless to the ground, splayed motionless against the dissheveled, scattered background.
Narcissa screamed a scream of pure agony, sobbing uncontrollably. "Not Bella...Not Bella too" "Draco and Bella".... "WHY??!" "SNAPE"! "I TRUSTED YOU"!
"I had to do it" reasoned Snape.
"YOU FILTHY HALF-BLOOD!" "HOW DARE YOU TAKE HER AWAY FROM ME"! "HOW DARE YOU"!
"Avada Kedavra"! shouted Narcissa. Snape, again, dodged.
"Expelliarmus" said Snape almost nonchalantly, attempting to hide his doubt.
"Severus, please..."
"I have to do this. You have no idea what he's capable of".
"Sev, PLEASE" begged Narcissa.
"You don't understand, he'll torture both of us to death. He'll kill Draco, too. I have to. For Draco's sake."
"I'm sorry, Cissy", said Snape.
He regrettably whispered "Avada Kedavra".
Snape now began to drag Petiggrew out from under the couch. Pettigrew attempted to launch a Killing Curse, which pathetically flickered out. He began to struggle intensely, kicking and screaming. He desperately tried to transform, but his Silver Hand hindered him. He tried desperately, spasming and convulsing, but couldn't.
"I-I-I-I"
Pettigrew blubbered, stuttering through his words.
"Save your breath"
Sneered Snape, walking over to his pathetic, writhing existence.
"Please, I won't say anything... I won't"...
"You can say that to Lily", said Snape.
"You aren't going to kill me, are you,?" inquired Pettigrew, shuddering and trembling uncontrollably .
"Give me one reason I shouldn't." said Snape.
"But...but..." pleaded Pettigrew
"But, but WHAT !?" said Snape mockingly.
"I DIDN"T MEAN TO"! screamed Peter!
"You didn't mean to". Oh, you "didn't mean to". You're stupid, but not that stupid. You knew exactly what you were doing.
"He would have... HE WOULD HAVE KILLED ME!" screamed Peter.
"Me, Voldemort, Sirius- We didn't agree on much. But let's get one thing straight- We all would have killed you." SECTUMSEMPRA!
A gash began to form on Peter's cheek as he screamed, rolling uncontrollably and spasming. Soon, it began to spread across his body, as he bled out all over the floor, gasping shrilly like a poisoned rat, until he was reduced to but a squeak, a dry husk of his pathetic self.
"Finally, that rat's out of my house" said Snape, with a newfound relief. "And hopefully I don't get any more of these Death Eater visits". I'll have to tell him an Auror found us".