My boyfriend says "I better not leave him...or else". What should I do?

I'm 25 and have an abusive boyfriend. We live together in his apartment. We met when I was 18 and he was 21. He(Nickolas) and my brother(Benjamin) are former good friends since High school (they were on a swim team together). I started to date my boyfriend when my brother recommended him to me for a date for a dance. My parents like him. He's very charming when we are out in public, but can be very threatening when we're alone. The main problem happened two nights ago (Its not the first of its kind). He was watching TV and I was finishing up cooking our dinner. The lights were tuned down so I couldn't see were I was stepping. I tripped and bumped into the book case, It wobbled and one of his swim trophies feel off from the top and shattered. My boyfriend got mad and said he was going to break something of mine. He grabbed a small plate my niece made in school for me. I got up and tried to grab it before he could, but he turned and elbowed me. I wasn't standing all the way up so him elbow hit my face. He looked back and started to laugh at me and broke the plate over his knee. I was getting up to leave, but he said "You better not leave me or else...". I was too scared so I just went to my bed and cried. The next day I went out to go grocery shopping and I ran into my mom. It was a bit odd because my parents live about 45minutes away and she doesn't shop for groceries that far away from home. She notice that my face was swollen and asked what happened. I broke down and started to cry. We went to my parent's home(were I'm living now) and I told them what happened. My brother Jake heard what I was telling my mom and he got upset. I told him to calm down, but then he went and called my brother (Ben). Nobody knows why Ben and Nickolas aren't friends anymore, but that they had a HUGE fight back in college. Nickolas went to the ER(emergency room) and my brother spent three weeks in jail. We've asked my brother what happened, but he never wants to talk about it. At the time I felt bad for Nickolas and took his side. My brother kept telling me he didn't want me to date him anymore, but I didn't listen (I wish I did). Up until now Ben didn't know I was together with Nickolas ,so he was furious when my brother Jake called him. Ben and my other brother Thomas are coming from Texas to help me move out and to teach Nickolas a "Valuable life lesson". As much as I hate Nickolas I'm still afraid of what my brothers may do to him. My brother said a few years back when I was one Nickolas's side the "If he EVER puts his hands on me, he'd beat him within an inch of his life ". My brother has always said "I never makes threats, but promises." In the past I wouldn't care, but now my brother has a wife, a little girl(my niece that made the plate) and a baby boy in about a month. My dad can't reason with the judge to lower his sentence. If they hurts Nicolas badly enough he could go to prison! My dad tried to reason with my brother, but he won't listen. We called the police on Nickolas before my brother gets here, but the swelling is gone, but My face still stings. My dad's best friend is a Police officer who said he'd try to talk my brothers down from doing something stupid, but if he can't he may have to arrest them. My brothers will be here on the 1st, what should I do, is there any other options!??