I don't own anything. Just so you don't get confused, this is from Snape's point of view.
Snape's Confession
I have done a terrible thing. I can never tell anyone, but it is horrible to live with the guilt. If anyone ever found out, I'd surely spend the rest of my days locked away in Azkaban, for I am the one who committed the crime Sirius Black was accused of.
Who told Voldemort to kill James and Lily? Me. Who helped him devise a plan for framing Black? Me. Who helped Voldemort to convince Pettigrew to join us? Me. Who decided they wanted out of it all when it was already too late? Me. I am a fool. Well, I was a fool, for too long. I can't say that it was all my fault. Honestly, I can't even say that I'm sorry for what happened. They deserved it, they really did. I can't say that there hasn't been a day that I haven't woken up in the morning and smiled knowing how much they're all suffering now. Then I think how much I'm really suffering, and how I can never show it. How did I ever get myself into this? I can easily answer that question, and I will now.
My home life as a child was never very good. My father was abusive for as long as I can remember. One day, when I was five years old, my mother had had too much. They found her dead. She'd used the avada kedavra curse on herself. I was left to spend the rest of my childhood with my father. He was a Death Eater. When I was seven, I watched as he killed a man. I promised myself that I'd never be like him. That was a promise I didn't keep.
When I was eleven, I got my letter for Hogwarts. It was on the train that I met the four people who would make my life a living hell to the point that I wouldn't be able to take it anymore. I had already known one of them. James Potter.
James had went to my grammar school, and for as long as I could remember, he'd always been making fun of me. I had always been his target for getting laughs. He always tried to be better than me at everything. The worst part, and I hate to admit this, is that he was. Not only was he arrogant, selfish, conceited, and good at everything, but everyone loved him. Grammar school hadn't been that bad. I'd had lots of friends in my crowd. But, alone on that train, with him being the only person I knew, it meant only one thing. Disaster. Of course, James had found three friends already. Their names; Sirius Black, Remus Lupin, and Peter Pettigrew. Sirius was always the so called "tough guy". Seemed he was angry if he wasn't fighting with someone. Remus always stood by James and Sirius. He'd been responsible for several of the pranks they pulled on me over the years. And then there was Peter. Little tag along Peter. Sure Potter, Black, and Lupin were friends with him, but he couldn't begin to compete with their talent. He was always the one who didn't do well in classes, while the others excelled. He'd always been the unpopular one, the one that everyone ignored. And when the time came to make him want power, he was easily convinced.
So, my years at Hogwarts were spent being constantly teased and embarrassed by those four. But nothing could compare to what had happened in our sixth year. It had been full moon. I noticed Lupin leaving and wondered where he was going. I noticed he went away a lot. They said he was sick, but no one gets sick that often. I asked Black where he'd went. He told me I should go down to the Whomping Willow, press the knot, take the secret passageway, and find out. And I did, never expecting what I would find. I had just entered the passageway when Potter showed up. He made me leave and go back to Hogwarts. But not before I'd seen what was in that passageway. Remus Lupin was a werewolf. He'd have killed me if I'd stayed in there any longer. That's what the four of them had been plotting to do. They couldn't just settle with constantly making fun of me, making my life miserable. Now they wanted me dead. Of course James made a big deal about how he'd saved my life, how he'd been brave. But really, he was a coward. Too afraid to suffer the consequences, too afraid to have a bad name. It didn't matter to him that he'd saved me. In fact, things went right back to the way they had been. I was sworn to secrecy about Lupin's condition. If I'd ever spoken of it, Dumbledore would have had me expelled. So I stayed silent.
After what they'd done to me, I wanted nothing more than to get even with them. They had such perfect little lives, with their friends and girlfriends and popularity with everyone. At least three of the four did. The other, Peter, was the one who'd help me achieve my goal. I wanted them to suffer as much as possible. I wanted the four of them to know what it felt like to be picked on, to be lonely, to be hated, to be in fear of their life. It was my turn.
A very powerful dark wizard, who called himself Lord Voldemort, was rapidly gaining power during that time. I broke the promise I'd made to myself and became a Death Eater like my father. I joined Lord Voldemort, and offered him money, all the money I had, to make James's, Remus's, Sirius's and Peter's lives as miserable as possible. He agreed to it, saying something about how he'd been needing to kill James, Lily and Harry anyway. And so, the plan was devised. We went to Peter and convinced him to join us. He agreed without much thought when Voldemort told him of the power he'd have. When the Potters realized they were in danger, they made Black their secret keeper. Voldemort told Pettigrew to trick Black into letting him be secret keeper. Pettigrew did a good job with that. It was at that time that I started to get nervous. I wanted out. I didn't want to work for the dark side anymore. I was afraid, so I went to Voldemort and tried to convince him not to do it. I told him not to kill Lily, James, and Harry. It wasn't that I didn't want them dead, I just didn't want to get caught and sentenced to life in Azkaban. Voldemort wouldn't listen. I left and joined the other side and became a spy under Dumbledore's instruction. But when Voldemort decides to do something, he does it. And he did. The plan worked better than we ever could have imagined. He murdered Lily and James on Halloween night, though he couldn't kill their son, Harry. Harry made Voldemort nearly die. But the plan still worked. The next day, Pettigrew framed Black, and they sent him to Azkaban without a trial. Pettigrew would be forced to spend the rest of his life as a rat because he'd faked his own death. And Lupin was left all alone, like we'd planned.
The plan had worked. It had worked better and more easily than ever imagined. And I was never caught. No one ever even began to suspect me of anything. They all thought I was on the good side, even Dumbledore. From then on, I was. But I have to live everyday knowing that I was responsible for the deaths of James and Lily Potter, that I was responsible for Sirius Black spending the last twelve years of his life as an innocent man locked away in Azkaban, I was responsible for making Peter Pettigrew spend the rest of his life as a rat, and I was responsible for taking everything away from Remus Lupin, leaving him to be what he is. A werewolf. Yes I am guilty. Yes, I feel bad about it sometimes. But I will never blame myself because they deserved it. They asked for it. They begged for it. Every time they pulled a prank on me, every time they embarrassed me in front of everyone, they'd made themselves all the more deserving. And they'd been too foolish to ever see it coming. The great, immortal James Potter had died. The champion had fallen. He had met his end. And now there is only one thing left to remind me of him.
Harry Potter, his son. Harry looks identical to James, except for his bright green eyes, which are his mother's. Harry is at Hogwarts now, where I am teaching potions. This year they gave that damn werewolf, Lupin, the Defense Against the Dark Arts job that I've been wanting for so long. I have to look at that damn kid Harry everyday. He is a constant reminder of what I did. He acts just like James, strutting around like he's better than everyone else, just because he's the 'boy who lived'. I wonder what he'd do if he knew that I was the one who made him famous? I wonder how he'd react. Of course I can't tell him. I can't stand to look at the kid. I treat him the way James and his friends used to treat me. He deserves it. Miserable kid. Just when I'd thought I'd never have to deal with what I'd done ever again, he comes along to constantly remind me. And he hates me as much as James did. But Voldemort is still alive and he's after Harry more than anyone. So maybe if I'm lucky…