I'm sorry to hear that. That's a really hard situation for you. Sounds like she has gotten postpartum rage, which seems to be linked to postpartum depression and anxiety. Please don't stop taking care of and defending your child as she grows up. When babies are small, they literally don't even know that they are their own person. They think that they are the same as the mother, so they mirror much of what the mother does. I hope that you pushing through and being there for your child as they continue growing up will help, so that the child has someone stable to rely on.
I would also try to get your wife into therapy. This is not her, it's chemical imbalances in her brain because of the pregnancy that is causing this. It is strong stuff. And while it is shitty, and does not excuse her behavior, I lean towards thinking that it is not her fault. She is basically sick.
I would try to frame it in a gentle way when suggesting it. Maybe not saying she is sick, but that you miss the loving partner that you know still exists, and that you both need professional help to figure out what the pregnancy did. According to this article the condition is entirely treatable. Good luck.
Can confirm. Step mother had 6 children. Is a massive narcissist.