Haha, this is awesome. You've given him a lot of depth, and an intelligence he often isn't portrayed with. And that's good, because I don't think he's a stupid monster, just a monster, and I like that you've kept that about him, not toned him down or filled him with remorse and regret simply because he had a bit of a crap childhood amongst his peers.
I like the reference to what I am going to cheerfully assume is Draco, being a pussy, because oh dear, Daddy's cause is too much for him.
And yes, the bit at the end - very chilling, very good. I can imagine Luna's entire stance and expression there, with her hand on her neck. I'm sure Fenrir would just smile nastily at her, or something equally unpleasant.