Is OTHER wrong in the post ? POST : So my step dad has been living with my mother and I for two years. He's 40 now, doesn't do the dishes, cook, complains if  we make food with too many brassicas, is way to defensive of his dog (who has literally bitten multiple children, including myself), listens to heavy metal music way too loudly (even if you ask him to stop) and if you try to bring any of these things up to him he throws a hissy fit and leaves. So I have a tendency to criticize him for his lack of doing things (or just doing things in a really strange way that either doesn't work or barely does), because I honestly feel like no one else does.

The problem is that he and my mother had a daughter last year (she's not the problem I love her). But I think my mom feels trapped with him, she's even admitted that if she didn't have my little sister the two of them would have split up a long time ago. She feels like she can't she can't leave him, and I understand why she feels like that and I don't think she should unless she really thinks she needs to. 

since you probably want an example: 

The other day, he put my little sister in her high chair and didn't buckler in or put the tray in front of her so that she couldn't fall out and proceeded to wheel her across the kitchen while she almost fell out. I told him the he should strap her in, and he started ranting about how I shouldn't tell him to how raise his kid, he knows more than me because he's older, ect. Even thought I spend more time with his kid than he does.

Our room mate (his long time friend) won't tell him when he doesn't shit wrong, and has the tendency to say "Oh that's just how he is". My mom sees that there is a problem, but whenever she tries to bring it up they either end up kiss (without actually resolving the conflict) or he gets all pissy and leaves.

So am I the asshole for criticizing him, even when I know he's not going to change no matter how much he needs to?