Am I right for disliking my stepdad?

First of all, I'm a 15 year old guy. My mom and dad got divorced when I was about 7/8 and my mom re-married a douche bag. He verbally/mentally abuses her, even got ARRESTED for pulling her hair out in public, but my mom bailed him out. He's a fat slob, always leaving messes behind and eating greasy *** **** on the computer and stuff. We've never really seen eye-to-eye. In 5th grade, I don't remember what I did but he came full on at me and called me a Pussy, ******, etc. I was effing 12 years old. I sobbed like a little kid since he was screaming all of these obscene words at me (wouldn't you). My mom tried to back me up, but then somehow it turned out to be my fault. Ever since then, I've been scared of him and have had 0 respect for him. Every time he's mad at me, my mom takes his side. ALWAYS. My sister (18) has already moved out to my dads because she was tired of their bullsh*t, but I'm too afraid to do that (she's 18, I'm 15). We've confronted my mom about him and she always defends him and after every big agrument "he'll change". My grandparents disowned my mom because of him, my mom hasn't spoken to her parents in 7+ years. I see my grandparents through my dad (and they aren't even related, but my dad thinks it's effed up for us to grow up without grandparents). But I'm not sure what to anymore. I'm literally getting emotional torn up. I just locked myself in the bathroom for a good 20 minutes and sobbed my eyes out (first time crying in years). What should I do?