But just because you can't justify something you did doesn't make you an evil person. It makes you human. And Peter is probably the most human character in the books, if you ask me. He's not perfect. He's light-years from perfect. He screws up. He's afraid.
That's the whole reason he joined Voldemort in the first place. Fear, plain and simple. I don't see any need to go to great lengths to try and conjure up some other reason. Fear can drive people to just about anything.
And it's not like Peter's alone in this, either. Look at the whole wizarding world! Most of them won't even say the guy's NAME, for crying out loud! So now Peter is suddenly a hundred times worse than the rest of them for being scared into Voldemort's service? Nuh uh, I don't THINK so.
Cut the poor rat some slack! He doesn't enjoy the whole Death Eater thing at all, that's plain as day! He shakes and stutters in front of his master and he never does a thing but follow orders. Even the orders he does follow, his reluctance to carry them out even Voldemort picks up on. Peter's not an evil guy. Think to the beginning of GoF, where he goes so far as to suggest to Lord Voldemort's face that Harry be spared: "It could be done without Harry Potter, my Lord," remember? He's not keen on seeing his ex-friend's son killed!
But then Lord V accuses him of disloyalty and Peter denies it in a high-pitched panic. Why? Because he's lying, of course. He's not loyal to Voldemort at all, he's just scared witless of him. Peter's trouble isn't that he's evil. He's just gotten in over his head and he doesn't know how to get out.
And he's completely on his own.
I don't think Peter's proud of what he's become. I'd say he more likely despises himself. He knows what he's done is wrong, gravely wrong, and completely without justification - even if he didn't cast the curse. Just look at his behavior in the Shack, even before Lupin announces his identity. He's in an absolute panic. He's done for. The weight of what he's done is so enormous that there's no way Black or Lupin's going to let him get away alive, and Peter knows it. What's the point, here? Peter knows the weight of what he's done. He has a conscience.
That doesn't mean he's suddenly brave enough to face a couple of executioners, though, and I MEAN executioners. Peter's not just going to out with the truth of things, not with Lupin and Black. Sorry, Peter, but the Truth won't set you free. It's going to be Avada Kedavra and that's that, no sympathy, there's no hope that Black and Lupin would ever understand. Why? Well because they aren't as easily manipulated as Peter is. They can say Voldemort's NAME, after all, they are not so controlled by fear. Black and Lupin think Peter should be just as resistant to fear and manipulation as they are, and when it turns out that he isn't, it's Peter's fault.
He's so... average. And completely without confidence in himself. He doesn't want to make the tough decisions, he doesn't want to be on the front line, he doesn't want the limelight and he's only trying to get by. Easily manipulated; doesn't know it when it happens and doesn't realize how susceptible he really is.
Peter's good-hearted and his intentions are honorable. He just fumbles.
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