Behavior expectations

My husband and I have been married for five years. The first year into our marriage he won physical custody of his two sons (now ages 17 & 12). We have a one year old son, and the youngest stepson is very aggressive with baby. He puts toy guns to the baby's head and say " I'm going to kill you." He pretends to physically box or throw punches at the baby.

My husband says that's just the way he plays, but I get nervous anytime he comes around to the point I don't feel comfortable leaving the baby alone with the youngest SS. Please help!