My nephew is horrid, mum wont listen, need some opinions..?

I know what i would do if this was my child, my mum will not listen to me, so i decided that if maybe i could get some answers from yall it might help. my mum has had custody of my nephew for 12 years (he is almost 13). his mother is in prison for murder (not our family) drs say he has adhd..he yells at her, screams at her, throws fits.. he has also threatened to kill her and my son while they are sleeping. he has hit my son and pets with big sticks, he has thrown his skateboard at my head. i have told my mum if he continues to act this way, i will call the cops on him and i dont care what she says. he doesnt mess with me anymore, and doesnt hurt my son. but he continues with the threats.. he acts out like this all of the time. you can be sitting there and all of a sudden start yelling for no reason. if you talk to him he will start yelling. he mouths off, has no respect for anyone. expects you to do for him what he will not do for you. do you believe this behavior is hereditary?