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Disclaimer: I have decided, now that I don't like Harry so much, I like Snape a little better.
I think Bellatrix was put in jail after Sirius, becuase in Dumbledore's Pensive, Harry saw Barty Crouch's trial, and Bellatrix was there, and Sirius told Harry he remembered the Dementors bringing him in.
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Dear James and Lily,
I am the last person you'd expect to write a letter, much less to my mortal enemy, James Potter. I heard what happened. And, even though I have never liked you, and never will, I am sorry for what happened. To both you and your little Mudblood wife.
Doesn't it just take the punch that your best friend betrayed you? I, personally, would like to believe it, but sadly, don't. We would know if Sirius was a turncoat. I am a DeathEater. Was. I am.
I am now the turncoat. Go ahead Potter, make your cracks about me being a yellow-bellied greasy haired coward. I am going for the good side now, and not just because He failed.
I am one of the only few, with the exception of Bellatrix Lestrange. She laughed when she heard that. She had been rather sober as of late, being of the fall of Him. I had never seen Lestrange laugh, and I hope to never again. It was cold and mirthless.
I hope that you rot in hell Potter, but if I know you, you will be smiling down upon me while I rot. And, don't worry, I promise to make your son's life a living hell here at Hogwarts. But, I promise, henseforth, if Harry's life is at stake, only once, I will save him. In not-so-humble thanks for you jumping last minute to save me from your beastly friend.
Greasily Yours,