I've been a psych nurse for almost 10 yrs and here's my take : this boy is def suffering from a mental illness , though determining what his diagnosis may be is not possible in this kind of forum and really doesn't matter at this point . What does matter is that his actions (throwing things , agitation) and wanting a gun to shoot himself tell me that he absolutely has the potential to be violent towards you, your baby and others as well as towards himself ( ie suicide. ) If I were you I would get myself and my baby out of that house -today. If you want to get him help, try to get him to an emergency room. Ask to speak wth the attending doctor or nurse practitioner and tell them about his behavior and your concerns for your safety and his. If he refuses to go , you always have the option of calling 911 and depending on the laws in your state he could be held for psychiatric treatment regardless of what he wants if he is a danger to himself or others- which unfortunately is sometimes the case for people who are very Ill and unable to make the best decisions for themselves. Google your state laws or call the emergency department and ask to talk to a triage nurse if you need to know quickly . Good luck
Maybe he suffered from some form off abuse growing up. Are there any suspicious family members or even any mental health issues withing the family? Check in on his school life it could be the effects of bullying, peer pressure,drugs or maybe even traumatic films or life experiences that he just hasn't told the family about. I'm a girl and I'm only 19 years old but sometimes I struggle to sympathise with people; like I absolutely love my niece and nephew but I would have flashes of myself hurting them and I'd imagine it feeling good but I'd feel sick at the thought. It's quite conflicting but its most likely down to the fact that I grew up with abusive family members and I had to suppress myself to deal with it. Like I had to become completely unemotional to not hurt them or myself in a fit of anger or fear. Maybe keep the younger kids away from him for the time being and try and get him to see a doctor. Weirdly enough it was my sister-in-law that helped me get medication and counselling and its really helped tone down the violent thoughts and reactions a bit.
This has the red flags of some sort of psychiatric condition, to me. Especially the hygiene issues. I would get him to a therapist as soon as possible- for his own safety and the safety of others around him.
I agree with Annie, he needs professional help and the sooner, the better. And from his comments of "don't touch me," it sounds like he has been molested. His failure at school could be due to the above or something like ADHD - either way, he needs help.
Move out then you will not have to deal with how he acts what he says or how he smells. He might be showing out for a reason.
He does not sound normal, are you asking if there is help for him or are you just asking if you should get out of there for the sake of your child? In your shoes I would move, it is possible that having your family added to the household is not making things better. Not that you caused him to have these issues but probably adding more people to what was originally just a one-family situation has its stresses for everyone who is there, particularly those who were there originally. Your husband should certainly encourage his parents to seek help for his brother.