I am further disturbed that he told you very matter-of-factly that he hit the baby "because I like to be mean" and that "yes, God made me this way." These statements of "fact", according to his sense of reality, indicate that he believes that he has no control of his violent behavior...this is who he is and how he will remain. The fact that he ascribes his meanness to God making him this way takes away any responsibility he has for these acts of cruelty and violence.
You need to network within your area mental health system to identify therapists who have worked with kids displaying these behaviors and lack of conscience. Given his reference to God, you may also want to have his therapy include some assistance from the clergy. These behaviors are not manifested in ADD/ADHD. Your therapist(s) should definitely factor in family dynamics when they treat him. Please get him into treatment as soon as you can and do not allow him to be alone with either of his siblings or animals. At this point, he cannot be trusted to control his rage and violent actions toward them. I will be holding a good thought for you all.