"This meeting of the Death Eaters is now concluded," Voldemort said. "You are dismissed."
Severus Snape left the grounds of the manor and disapparated, reappearing in Spinner's End.
The time-traveller watched Snape reappear at Spinner's End and walk towards his house.
He never reached it.
Severus Snape never saw the Killing Curse that hit him in the back, extinguishing his life.
He'd had his second chance, so very long ago, and he had chucked it out the window like so much trash. He would not get a third one.
One Death Eater down, who-knows-how-many-more to go. Rookwood would have to be next: as an Unspeakable at the Department of Mysteries, he was the most dangerous of the Death Eaters at the moment. He could potentially figure out that a temporal displacement had taken place. He had to be silenced preemptively, before he figured it out - and he may already have figured it out.
Dolohov's death would gain the Weasleys' support; Pettigrew's death would gain that of Sirius Black, assuming Black wasn't furious not to have been the first to kill Pettigrew. (Which was actually too big of an assumption to make, come to think about it.) The problem was people like Dolohov, the Lestranges, and the Carrows were too flat-out dangerous to face in person. A more subtle strategy would need to be used with regards to them.
And if Rookwood had figured it out, killing him may not be enough to shut him up. He could come back as a ghost, and there was no way to bind his soul - well, not any that the time traveller had knowledge of.
The Resistance - or what was left of it, anyway - had decided to Cruciate Voldemort until his mind stopped working when it had become clear that he couldn't be killed. They hadn't known of any soul bind that would imprison him after they killed him, because if they had, Voldemort would be dead, not a drooling shell on the floor in a thirty-feet-underground chamber in 2011. But the Cruciatus Curse had done what it was supposed to do.
Besides, parading Voldemort around in the middle of the Diagon Alley - or what was left of it, anyway - gave the Death Eaters proof that their master had been defeated. They surrendered in droves - and, without preamble, had promptly been massacred by the remainder of the Resistance.
It was ugly, horrific, and might not even have been necessary, but it had happened anyway. Because what people do, when they're scared, is lash out. The Death Eaters were scared of the Muggles and Muggle-borns, and so they lashed out at them. Now, the remainder of magical Britain's citizens were terrified of the Death Eaters, and lashed out at them.
Maybe amnesty wasn't an option. Maybe rehabilitation wasn't an option. Maybe the Death Eaters were all just wild animals. Maybe the Third Massacre of Diagon Alley had had to be done, and the Death Eaters had all needed to die. Maybe.
What was damning was most of the Resistance members' sheer glee as they killed the Death Eaters, used their own Dark curses against them, tortured them into insanity, made them beg. They were laughing.
No one would ever need to descend to that level if the time-traveller's plan worked. If it worked, countless generations would be spared the suffering of the Second Wizarding War.
Time to make an exit.
The first person arrived on the scene ten seconds after Snape's killer had Disapparated. That person was a Muggle, and didn't understand why Snape was dead. It was as if the dead man had simply dropped dead for no reason. There was no cause of death that would have made any sense.
Then the Aurors showed up and Obliviated him of the last three minutes of his life.