Adoption?

So we recently got our 13 year old nephew placed with us. I have a son that's 12. We are in the process of adopting our nephew. But... everything his case worker told us was a lie. Our nephew lies, steals, hides things, refuses to do homework or even work in class. He hides my sons things all the time and lies. Its getting so frustrating. We take away his xbox because of his grades and he gets so depressed and wants to go back to his old foster family. Then when we give it back he's fine again. I have heard some pretty crazy things he has done to his past foster families. And we're really having 2nd thoughts about this. But I feel bad because he calls me mom and says he loves me all the time. I have grown a soft spot for the kid but would I be the worst person ever to not take him? Has anyone else gone through this before?

EDIT**

Thank you everyone for your comments. Im not trying to be a shitty person I love the kid but I'm scared for my own sons safety also. I didnt even tell half of what he's done. He keeps trying to sabotage my son in anyway possible. He has hidden his school work and school laptop so he won't get good grades. He hides his clothes so he can't leave. He hides his electronics. He also made a comment when my son was at a friend's house that it feels like my son never existed and it would be better if he was the only child over my son. He always asks me how our life would be with just him in the house than my other son. It seems as though he wants to be the only child and im scared he's going to hurt him one day. He was watching my son sleep one morning and thats creepy. I've always wanted to adopt because I can't have anymore kids but this is just getting weird.

Also he was seeing a therapist but he just sat there the whole time and never said anything. He went for a year before we got him.

He also lied about a relative molesting him so its stuff upon stuff I have to worry about.