She's probably sick of hearing from me too...lol...but I worry a lot about her and her kids. I think abusers are pretty much psychopaths. Plus this kid is unsafe to animals and plays with fire--two big red flags. He has no remorse. What is one who abuses people and animals if not a sociopath? He is very controlling and calculating about what he does too.
I don't think his behavior will stop if you leave. I think he will continue to steal, get into fights, lie and abuse with daddy's blessing. He may sexual perp too--a big fear of mine--but we talked about how you can try to protect your stepdaughter from afar. Kids perp on their own sex too--and that could be your son.
I think it's a good idea to make plans, but I wouldn't take too long to do it. This child isn't getting any younger or any less dangerous. He is one of the most disturbed and dangerous kids I've read about here in all my years on the board and easily has the father most willing to pretend he's just a "poor kid who had bad breaks." To me, Dad is a big part of the problem and will be excusing him even when the kid finally comes at him with a knife. And with his knife obsession he will. He isn't storing them so he has an excess of silverware. They are weapons to him. He slashes things. Maybe next he'll start slashing people. He is sly, smart and conniving and the type of kid you'd never want near your kids in other situations. You married his father, but his father has proven he can't be trusted to know when his child is a threat.
Call around, make plans, get out. If it's both of your homes, maybe you can make HIM be the one to leave. Then he and difficult child can live unhappily ever after. Either way, you and yours don't need this and can't control it. husband is so disappointing.