First and foremost be honest. Honest with yourself and with him. Don't treat him as a child. Be an adult, talk to him about the reason why you want to break up. Think long and hard in it before you do. Practice the conversation you are going to have with him so that it will be easier for youu to tell him without getting confused or too emotional. Going in with a calm demeanor will go a long way.
My initial advice is this: If you are in physical and/or emotional danger leave the relationship immediately and seek appropriate treatment for injuries and emotional turmoil. If you need shelter, money, food, and the like, find an emergency shelter or locate a place of worship. If, on the other hand, you have adequate resources available, use them and begin to heal. There is no shame in recognizing that a relationship is caustic and unsustainable. If you need police, medical, or legal help, seek out this important resources as soon as you are able.
I have found that the best thing to do when you feel you have to breakup with someone is to be kind and not get into too many details. Just say that it’s something you need to do and you can give a little explanation if you want, but usually that doesn’t make it any easier. Short and quick is the best way to go, in my opinion.