He cast a spell on the Whomping Willow and ran faster than he knew was possible through the tunnel. He could already feel his temper rising at having to once again chase after this child.
He arrived at the Shrieking Shack in high dudgeon - just in time to see green light flash. His heart stuttered in his chest and he gripped his wand so hard, he thought it might break. He forced himself to breathe before entering.
He blinked when he discovered Remus Lupin and Sirius Black pointing their wands together at a spot. He eyes tracked where they were pointing. His wand almost dropped to the floor when he discovered Peter Pettigrew still alive. He also noticed all the children were fine, though the red-headed dolt seemed horrified.
"You were the Secret Keeper," he heard Sirius say. Peter started trying to make excuses.
Snape surprised himself. He couldn't help it. He started laughing. "Oh, surely even the arrogant toerag James Potter wouldn't be stupid enough to trust a rodent like him."
"Well, they had planned on asking you, but didn't trust that you would say yes. And they thought he would go unnoticed." Lupin's dry remark did make Snape drop his wand.
"You're not very funny, you know," Snape said, scowling and leaning over to grab it.
"You were going to be Secret Keeper, I was going to be godfather in a few months. Life is unfair and full of disappointments for everyone." Remus stared at Snape calmly, no hint of laughing on his face.
Snape felt himself blanch. "They decided that having a powerful wizard hunting them down was a good time to - "
"Maybe now is not the best time to discuss this. Especially since it never actually happened," Lupin said, obviously annoyed now.
Snape was reeling. He looked at Sirius. Sirius shrugged. "Didn't know about the godfather bit, but I did hear them mention you as choice for Secret Keeper, once they decided it wouldn't be me. And I also know they decided on Peter. That was James," Sirius said, addressing the boy. "Loyal to a fault."
"A fatal fault," Snape pointed out, sparks shooting out of his wand, glaring at Peter who squeaked and tried to hide. Snape was unsure how far his anger might have carried him if the Dementor hadn't found them.
There was a chorus of "Expecto Patronum!" The Dementor was stubborn - for some reason, it came to Snape who had been reluctant to point his wand away. He was not pleased that the boy was actually able to cast a patronus. He was even less pleased to see his doe joined by a stag, though the Dementor did go off into the night. There were several moments of quiet.
"A doe, Severus?" Lupin asked, his eyes wide.
"I only know one other person with a doe patronus," Sirius said this, gears in his brain clearly grinding. When he got no response besides Snape's most serious sneer, he went on. "Just as I only knew one person to produce a stag. Whose was that?"
The child, somewhat uncertainly, raised his hand. Of course. "Just like James," Remus noted.
Snape was the one to notice the rodent trying to crawl away. He took great pleasure in casting sectumpsempera on him and seeing Peter fall to the ground, a bloody mess. "That was a bit much, don't you think Severus?"
"He is responsible for the death of the boy's parents and the rise of the dark lord and you question my punishment? I would have thought you might have cast a worse curse." Snape cooly replied to Lupin.
"There are children present. Besides, it's not really my decision. Or yours, come to that."
"Are you really going to ask the boy what should be done? Why not just take them all to the pub in Hogsmeade and order them fire whiskey?"
"Maybe you would do better with a bit more fire whiskey," Sirius muttered. Remus and Snape both ignored this.
"Harry," Remus said, not taking his eyes off Snape, "I feel you should have some say. It was your parents he sold out to Voldemort. What do you want to do?"
Peter's groveling did not impress the boy. But Snape could still see the moment Harry decided. His eyes were exactly the same as hers. "Let's take him to Dumbledore. He'll set things right."
They marched back. Snape kept his wand pointed at the back of Pettigrew's neck. He even "accidentally" stabbed him with it once. The resounding squeals and resulting glares kept him from further jabs, but not from trying to burn a hole through Peter's head with his mind. It didn't work. He couldn't read Peter's thoughts. It didn't even make him feel better. But it did make Peter squirm, so there was that.
Severus Snape had the satisfaction of watching someone else be on the end of Dumbledore's disapproval. He was irked to see Sirius go free, but not surprised. He was pleased to see Peter dragged off to place rodents like him belonged: in the cage of a prison called Azkaban.