My mom and her bf constantly fight and it’s scary

Throughout the years my mom and her bf always had there issues where they’d fight and it’d lead to my mom physically abusing him and him defending himself but recently it’s gotten worse. For the past few months her bf has been on serious drugs and he’s been snapping at her more recently to where he threatened to lay hands on her and to where there most recent fight I woke up to him beating her. I literally don’t know what to do, it’s so scary having to hear this and ik I could leave since I’m almost an adult but also idk if I want to since I have a little brother and I don’t want to leave him to be the one to deal with this. It’s all too much.