What's bothering my brother?

Negativity is attacking my brother's mind. He's diagnosed with a form of autism and ODD. My brother has been really negative every day, it's not good to be around negative people. He don't want to go anywhere. Anytime my dad mentioned the doctor, my brother hits his head and says that he's being traumatized. The negativity is making my brother say things like "go die". He acts out of control and misbehaves whenever he can't get his way. He's short-tempered at 18 years of age. And he says that he said that out of anger. Now how do we help loved ones who act out of control? And how to limit their actions that make us want to be angry as well? Dealing with loved ones, or people we know that act like this? How do you all deal with people like this? Understand them and show compassion and understanding? Be mindful and use radical acceptane and wise mind? Whenever we mentioned that he needs help, he flips out and turns into an angry man. I'm sure other people have experienced these kind of loved ones, especially siblings.