Okay....I'm gonna play the devil's advocate here...

But people search stupid "dark" stuff all the time. I Google "How to kill your brother" but it's just for humor and I really love my brother.

He is still young and you're the mature one in this relationship. I think you do understand that it's difficult for your husband....but it's also very difficult for your stepson....that he has no place to call home.

Young people are verbally violent by nature....are opnionated...feel like they need to fix someone....say things just to hurt you (because deep down they are hurt). But you do see that he's making some improvements.

Just support him to go to therapy and help your husband help him to be independent.

Generally people turn killers hurt animals first. Has he ever tortured an animal?? What are the other reasons you feel he can hurt you?

I think you should go to therapy yourself and discuss this with your therapist. We all don't know about psychology to give any useful answers.

Maybe your gut feeling is right........that way you can ensure that he gets help.
Maybe your gut feeling is wrong.....that way you can avoid hurting someone emotionally. Accusing someone of something when they are no way close to it can leave damage of a lifetime....and he still is very young and has mental health issues.

All the best.