Moms boyfriend...

Okay so I'm 15, and I usually get along with my moms boyfriend. He's lived with us for 2 years and he is usually fine. But lately I just don't like him. He treats my mom like shit.. Usually he is great he's respectful to my mom but when he gets mad he's over dramatic. When they fight he calls her names like bitch and cunt. Sometimes he tells her how worthless she is and literally every time they fight even if it's over something stupid like how he's always on his phone he'll threaten to leave her. I really wish she'd let him go. This has been going on for about a year now and I'm sick and tired of it. I've even told my mom to let him go but she doesn't. And tonight I tagged my mom in a post on Facebook about how it's a pregnant women and there is a little cartoon baby in her talking about how much she loves her mom and is thankful for everything her mom has sacrificed to make her. I tagged my mom in it basically saying thank you. And her boyfriend said that I probably tagged her in it because I'm obsessed with babies and pregnancy and it's totally not normal and went on accusing me of basically wanting to have a baby too much. Okay. So I will admit, I love babies and pregnancy. I think it's the coolest thing ever. BUT that might be because I want to become a midwife and deliver babies and help women through pregnancy because I'm fascinated by it. Not because i want to be pregnant. I just want to know other people's opinions on it.. Maybe I'm just a kid and am mad because of what he said about me or maybe he actually is verbally abusive. Idk. Thoughts? What should I do?