AIO for threatening to move out?

Last night, my 'brother' threatened to hit me with a hammer, then used it to smash part of my door in. All because I tried to clean the living room which was full of his junk he refuses to do anything about.

That wasn't the first time he's done something like that.

Throughout the years, he's broken multiple things, ranging from doors, windows, and electronics. He's bit me and even purposely ran into my ankles with his bike and scooter when were kids. Never cried when our grandfathers, his hamster, or our dog died. And always demands my mother cook him food. 

For context? Here's the kicker: He'll be 19 in March. So we ain't talking about a little kid, here. 

The tantrums he's caused has given my mother heart attack scares, my aunt even worse problems with her blood pressure + back, and given me God-awful attacks to the point where I feel lightheaded. 

While my aunt wants him gone (even told him to drop dead once), my mother actually, genuinely wants him here. I've expressed my pain and feelings over this, yet she's always dismissive. Instead, she goes on about how SHE'S also a victim here. How he affects not only me, but everyone under this roof. She never once took the time with me to sit down and talk, like you'd expect a parent to do. Oh no... she always defends him. 

Last night was the last straw. I'm not even going to bother asking her if he's still going to live here. Because I already know the answer. And for some reason, it hurts. 

It hurts like Hell. 

So I gave her an ultimatum: Either she kicks him out for good, or I move out for good and cut off all contact with them permanently (maybe aside from my aunt, I don't know). Basically disowning them. Never inviting them to my wedding, etc. My mother's reaction to this, was that she's "a bad mother", and she "might as well not work".

As I don't have a lot of money, I'm currently trying to get into contact with other extended family members who'll hopefully hear me out and take me in. Of course I'll pay for rent with what I have whilst looking for a job (because if they do let me stay with them, none of them live in the same city so I'd need to look for work), and will pretty much just owe them for life. Otherwise, I'm also looking into homeless shelters and roommates. I mean... we have dogs I don't want to leave. Which is one of the reasons I stayed so long. But I'm also at my breaking point. 

So am I overreacting when I said I'd move out?