Wow, what a fucking loser this guy is. The fact your mother is dating him says a lot about her as well, I am sorry to say.
Are you staying with your sister? Because you should be. EDIT: Sorry, I just read the rest of the thread. Nvm.
No. Wrong wrong wrong. Your mom put herself in this situation by staying with an abusive alcoholic for the sole reason of financial stability. She put YOU in the situation of reacting as an 18 year old would, particularly an 18 year old who has been victimized just as she has. Bunny, stop being the victim NOW while you are still adaptable. If you don't I'm telling you your life is never, ever going to be as much fun as it could be. I know you're 18 but you admit you're not mature and you're right. Stand up for you and stand up for your mother because she obviously can't. If you don't you're bound to end up just like her, constant chaos, tied to someone just like your step-father. I'm not spouting opinion, this is simple pathology.
You very likely saved 2 lives, your mother's and your own. Try to remember that when you start to feel down, Bunny. And you probably had a lot less to do with him leaving than your mother is letting on right now. You said he was gone when you came back. Something tells me your mom likely finally booted his ass and now she's just worried, so she's piling it on you. Don't feel guilty at all, you probably saved her a lot of trouble for when you do go off to college. What if he came in crazy drunk one night and you weren't there to call the cops if things got out of control because you were away at school? I'm sorry, but struggling to make ends meet and doing without is a lot better than living with someone violent. Your mom doesn't know how LUCKY you both are to be alive and how good she truly has it and that's a shame.
Don't blame yourself. I blamed myself for a long time. Still blame myself sometimes, and all it does is cause me sleepless nights. Even if she does point the finger at you, you need to know in your heart of hearts you did the right things. Even if she blames you, you saved yourself and her. She will thank you eventually, even if she cries now.