I also agree that you having the children more would help Mom if she is indeed overwhelmed, I do not like the violent stuff children are seeing and I do feel it affects them if only when they are small .I would try to curtail him seeing it ..good luck ..
I agree with Sandman. I don't think it sounds like a situation where the child needs to be removed, but I do think the parents need more help. I would offer to help parent and care for him frequently.
I think the problem is at least partially in the fact he has a little brother and sister at home too. The mom's got to be going nuts with a fairly new born and another not a lot older. He is not getting attention and is starting to seek it. Yes, the parent could use counseling (don't know if they give it for running a 3 ring circus though). And taking him out of the house completely is probably not fair to him.
Perhaps a partial combination. Offer to take him for a couple days at a time. Or maybe in the afternoons or mornings, so his mom gets a bit of free time and he gets attention.
His mom really needs some advice on working with him and the other kids. There actually is a lot she could be doing. Maybe get her to post here asking for suggestions on ways to help him (and her). I don't think your initial post would be upsetting to her. Heck print out the post and show it to her. If you are worried about that - I'm pretty sure that we can make these comments disappear if it looks like she may post.
Is the problem the stepson and his x-box or the parents' treatment of the son? Plenty of kids have x-boxes and kids love zombies and other weird creatures. They know it is fantasy. The mother has already lost custody of one child. Perhaps she has few maternal skills. Probably the parents needs counseling. If that is a nonstarter, the poor boy might be better off with his grandparents.
It sounds like he ios learning this behavior and the thoughts are not good so if the stepbrother is playing with zombie games this is where the problem lies and will not change .Its up to his parents to stop step bro playing with stuff like that .little bro is learning from him ,.