How do i deal with my three year old step son.?

when me and my husband were dating i thought that this kid was just the greatest quit and well behaved and did almost everything you ask of him. after spending more time with him i noticed that he would play the stupid card with his father to get out of trouble if he did something wrong. this would really get to me because my husband doesnt let my two year old get away with the same type things. after allot of patience pain and tears we mostly get through that but then i started to notice that he would intentionally hurt my daughter. one day he was beating her with a toy in the head, hitting her in the head with the top of the toy box and and then i caught him wrenching her leg where i thought he was going to break it. i almost lost it on him that day. after a while i thought we were over that too untill recently. he now is more sneaky about what he does to her. like lately he tries to coherse her to do things that he knows is wrong then tells on her for it and laghes if she gets introuble for it. also i have cought him the last few days spilling things on the floor and trying to get her to copy him to get her introuble. then just today both of the children were told not to climb inside the toy box. when i came back n the room to check on the kids i saw my daughter sitting on the lid while my stepson was telling her to get down and open it. when she told him no he reached out and slaped her across the face. mind you they were just introuble for playing inside the toybox he was trying to do just that again. when she was trying to be good and told him no he slapped her across the face. after he got introuble and i asked him why he did it he told me with great confadence and with a bit of a smile it was because she told him no. please help me out here how do i cope with a child that continues to try to hurt my own child and get her into trouble when she is doing nothing wrong. i first thought that it was because he was jelouse of my relationship with my daughter. so i started to spend allot of time with both of them together and even split up time for them individually. this did not help. he still continues to treat her horibly. then i thought maybe he was mad at me and taking it out on her because i was taking daddy time away from him but then any time my husband would try to spend time with him he would want to go play with my daughter instead. and finally i thought that maybe if my husband and i spent some time alone with hie without my daughter then that would help. but i sent my daghter to her fathers for a week and instead of improving his behavior my step son just became defiant and rude then ever before. now it seems like he is getting out o controll and i feel as though i will not be able to controll my anger if he continues to treat my daghter this way. i told my husband that i can not deal with this and i will not have hime living with my children if he continues this. i also have a four month old and i am afraid that he may have this kind of behavior torward the baby as he gets older. what should i do to keep my family together without someone getting hurt.