my son who we adopted

our son we adopted was in a horrible living situation and born at 32 weeks. we adopted him when he was 4 , but he is contstantly hurting people and tells them he will kill them. He has stated to me "you are not my mother you ******* *****" and "i am going to kill you chop you up with a ginsui knife while sleeping. He has siblings 1 brother and 2 sisters which we adopted. They all have medical conditions but NOTHING like this. We are at our wits end with going to doctors to no avail. please anyone someone Help us help him.