How much rough housing is normal for two brothers, or do they actually hate each other?

Keep things safe. One important thing is for families to have clear rules about hurtful behavior. It should not be allowed. You need to take a more active role in teaching and limit setting. Your children may be developing very bad habits.
Sibling rivalry. We all know innocent sibling rivalry exists. But when it gets to a point where there is either physical or emotional concerns then one should be worried. I don't know ages of your children, if they are old enough, you can sit and have a talk with them, either alone or together.
Behavior probs. With raising our kids, we did not tolerate any behavior if it crossed the line and became violent. I published a book on kindle called parenting the kids according to a child psychiatrist which could really help you. I suggest doing whatever it takes, to stop the violence, if it is going on in your home. We never used physical punishments with our kids.