From my pov, it’s not that much about letting them get away with murder.
It’s about being aware that life is never black and white. And if it was, I’d be in trouble too - I’m very aware that I’m not perfect either.
It’s about being aware that a separation is never as easy as ‘divorce=end of all problems’. When you have children affected by the behaviour of your DH, there is a question of what will happen when said children spend time in their own with their father.
I remember a woman on here who had an awful time separating from her autistic dh. He was intent on getting custody 50/50 (as is his right btw) but was also completely unaware of danger and his child’s ability, putting the child’s life in danger. I’m sure handling the relationship between your dd and your narcissistic ex can’t be easy either- for you and her.
There is a question to ask around the limit, the tipping point where staying is worse than leaving (for the children) but where that limit is is never as easy to see.
And then you have the whole past experience, feeling responsible, ashamed, guilty that you are in that situation. Again, there is a reason why women find it hard to leave in case of DV. I feel some of those reasons apply to us. They certainly have applied to me. And no amount of ‘why on earth do you put up with it?’ will break those barriers.