Author's Note: This is just a short little plot bunny I had to get rid of. This scene takes place at the end of Harry Potter and the Prisoner of Azkaban. This is also assuming that Dumbledore has already gathered his trusted colleagues (at the very least McGonagall and Snape) to tell them about Sirius' innocence and how Peter is still alive.

Regret

"Severus. I need to ask you something."

Remus Lupin was leaning in the doorway of the Potion Master's office, feeling too tired to keep himself standing of his own accord. The transformation the other night had taken a lot out of him. Since the Wolfsbane Potion had been created he hadn't had a transformation that had gotten so out of control.

Snape had his back to him, sorting through some books on his shelves behind his desk, packing some away in a suitcase that he would presumably be taking with him as he left the school for the summer. He glanced at Remus, regarding him distastefully for a moment before turning back to his task as if he weren't even there.

Remus took a deep breath as he stepped more fully into the room. He would be leaving Hogwarts tonight and may never return. This could very well be his only chance to get the answers that he desperately needed. "Severus, please."

Snape whirled around, his eyes cold. "If you think you can swindle ingredients for the Wolfsbane Potion, you are sorely mistaken, Lupin," he spat. "You are no longer a teacher at this school and can no longer hide under the Headmasters coattails. I owe you nothing."

"That's not what I was going to ask," Remus said patiently. He hobbled forward so that he could place his hands on Snape's desk in order to balance himself.

Snape glared, his eyes narrowing suspiciously. "What is it."

"Something has been bothering me," Remus started slowly as he studied Snape's face carefully. "Something that Sirius said the other night."

Anger flared in Snape's eyes. "Black is still a wanted criminal," he said in a low voice. "If you're helping him…" He let the threat hang dangerously.

But Remus wasn't fazed by this. "I'm aware," he said calmly. "I was just thinking about something he said the other night in the Shrieking Shack. About how Peter wasn't hiding from him, but rather from Voldemort's followers who might blame him for their Dark Lord's downfall."

Remus paused, seeing how Snape would respond to that.

"So?" Snape finally said.

"So," Remus repeated slowly. "That's a bit curious, don't you think? He makes it seem as if Peter being the one to betray Lily and James is fairly common knowledge amongst the Death Eaters."

"If you have come here expecting me to rationalize the ravings of a madman, you are loonier than I thought," Snape growled. But something uneasy flashed behind his harsh gaze. It seemed he knew where Remus was going with his train of thought, but he certainly wasn't going to help him get there.

Remus met his gaze. "It makes me wonder how our spy in Voldemort's inner circle had no idea," he finally said bluntly.

There was silence for several long moments and the two locked gazes, each daring the other to look away. Tension grew. Finally Snape turned away. "I don't have time for this."

Remus could only stare for a minute, his mouth agape. In refusing to directly answer the question, it was clear that Snape had given him his answer. "You knew?" he demanded, dumbstruck. "You knew and you didn't…" He shook his head, unable to complete his thought. "I know you had no regard for James, but Lily—"

"I never would have put Lily Evans in danger," Snape snapped as he whirled back around, his eyes flashing dangerously.

"Potter," Remus said quietly.

"What?" Snape spat.

"Lily Potter," Remus corrected, a dark note in his voice. "She and James had been married, if you'll remember."

Snape glared before he turned away again. "Get out of my office," he said lowly.

Remus studied his back for a moment. Given his reaction, he did in fact believe that Snape had not known that Peter Pettigrew would hand James and Lily's location over to Voldemort that night, at least not in time to stop it from happening. In truth, he had expected from the beginning that that thought had been flawed. But that wasn't the only question that troubled him enough to bring him down to the dungeons that day.

"Even if you didn't know in time to save them, it seems that the news of what Peter had done spread very quickly among the Death Eaters," he went on determinedly. "Given your position, I would imagine that was how you learned what happened that night."

"I really thought by now you were be smart enough to leave when you knew you are not wanted, Lupin," Snape growled, his back still to him.

"You must have known," Remus said, his voice rising with an accusatory note. "You must have known after Voldemort fell who it was that had actually betrayed the Potters. You must have known that it was Peter and not Sirius!"

A ringing silence followed this revelation.

"If you will recall," Snape drawled, not turning around, "Sirius Black was not imprisoned for murdering the Potters. He was sentenced to life in Azkaban for murdering a street full of Muggles along with Peter Pettigrew."

Remus was shaking his head though. "If you will recall he was also charged with conspiracy to murder Lily and James Potter!"

"And if that one charge had been dropped it would not have changed his sentence," Snape shot back, whipping around again. "Mind you, he would have had to actually have had a trial for that to happen. Since he did not it seems hardly worth agonizing over minor details, don't you think?"

"You could have come forward," Remus insisted, almost desperately now. "You could have told us that Sirius hadn't been James and Lily's Secret-Keeper, you could have told us that it was Peter who had betrayed them. If we had known that we could have fought for a fair trial for Sirius, he could have been spared his terrible fate!"

"How was I supposed to know that Pettigrew faked his own death?" Snape hissed. "How was I supposed to know that it was little, witless Pettrigrew who had obliterated a street full of Muggles and not Black flying off the handle as he so often did?"

"But you knew that a man who had gone after the Death Eater that betrayed James and Lily Potter would have been treated to the benefit of the doubt!" Remus almost shouted. "He would have been given a trial and been given a chance to prove his innocence! Instead he was treated like a Death Eater and was thrown away without a second thought from anyone! You could have prevented that!"

"It has never been – nor will it ever be – my responsibility to clean up Black's messes," Snape said evenly.

"But…" Remus started, but honestly couldn't think of what to say to that. He really hadn't expected Snape to be so cold.

And then, inexplicably, Snape smirked. "And if you were such good friends, perhaps he would have trusted you enough to tell you about the switch. Then you could have done that yourself. Instead, Black let his arrogance get the best of him once again." He seemed about to say something else, but stopped short.

However Remus could clearly see the words that he had not said. He gaped at him. "You… you think he deserved what he got," he said softly as he slowly came to the realization.

Snape turned his back as he began calmly going through his books again. "Leave."

"He spent twelve years of his life wasting away in a prison for a crime that he did not commit," Remus pointed out, feeling his heart twist in his chest.

"Perhaps," Snape said coldly. "That does not mean he didn't do other things to deserve what he got."

Remus stared, wide eyed. It took his mind a solid minute before he was even able to form a coherent thought. "You allowed an innocent man to rot away in Azkaban all because of a childhood rivalry," he said, shock and anger fighting to be heard in his tone. "That's a new low, even for you, Snape. I never expected you to be so cruel."

Snape didn't spare him backwards glance and Remus knew the conversation was over. There was nothing more for either of them to say. Remus straightened up, but he felt heavier than he had when he walked in. He took a moment to gain his balance before he turned around and limped out of the office, closing the door carefully behind him.

Once he was in the privacy of the deserted corridor he leaned against the wall heavily, his head spinning. All of this could have been prevented. The pieces had all been there. He remembered all too vividly the days after Lily and James had been killed. He had been in shock; unable to comprehend what had actually happened. At first he didn't want to believe that Sirius had betrayed them. He had gone over and over it in his head, trying to find a way that would explain why three of his friends were dead and the fourth with in prison for murdering them. But the entire world had believed Sirius Black to be a cold-blooded murderer, true to the surname that had haunted him all his life. Eventually it had just easier to go along with popular belief.

Now that he looked back on it, knowing what he knew now, he couldn't believe that the truth hadn't been obvious. Sirius had been one of his best friends. He should have known. He should have questioned what happened that night, fought harder to uncover the truth. He would regret that deeply for the rest of his life.

Things could have been different…