Husband taking up for his baby mama to me

So my husband’s baby mama tried for a whole year to break us up when we first got together. Well this was 3 years ago and we “get along” now except my husband says he hates her but is civil and stuff. I’m civil with her too. But my stepdaughter was acting bratty and stuff and my husband said “well maybe ____ is making her like that” so I said “she could be” and literally as soon as I said that he got defensive. He immediately goes “well how (my name)? Tell me how ___ could be making her act like that? Give me one example. You just like to try to blame everything on her and make it all her fault. How could it be ____’s fault?” So I said “idk” just backing down. And he literally freaks out and makes me give him one reason why it could be her fault. Like WTF? HE WAS THE ONE TO EVER EVEN MENTION It BEING BECAUSE OF HER. Idk but now I’m feeling some type of way over here... Why is he defending her? This isn’t the first time he’s defended her to me either lately.

Edit: He’s the one that’s constantly saying how crappy she is because she won’t take her kid to the doctor when she’s sick or make her go to bed or whatever. And claims he hates her. Of course I hate her but I’m civil with her. Sorry but he shouldn’t be defending her. She’s not his family. She’s his baby mama who he claims to hate. We’ve talked about her before and he’s never said anything until lately.