I can look at my prior M now and know there were problems - I was codependent, and his is likely NPD. Stupid thing is, it was working until her involvement. Working in a healthy way? No, of course not. As another site says, water seeks its own level, and each of our dysfunctions meshed / enmeshed well.
I also recognize that if it wasn't her, it would most likely have been something else. It could very well have been me who grew dissatisfied at some point.
But I do believe that she was the actual catalyst for my M ended how and when it did.

Has she ever done anything that is harmful to the kids?
Well, destroying two families is not to be overlooked. But that isn't it.

I have never seen her as a good or competent mom to her own kids.
I saw her slap her D(then 13) across the face and throw her out of the house into the snow with no shoes or coat, locking the door behind her. The girl ran away to a friend's house (picked up later that day by her dad).
This same girl now has multiple piercings and tattoos, and just turned 17. She is allowed to date an 18 year-old drop out who has many stays in social and psych institutions.
I was witness to how she trained her little ones to be terrified of sirens. When they were within earshot, she would turn to them serious and acting scared, telling them "they are coming to get you!!" The kids would completely loose it, trying to run and find someplace to hide, sometimes even hurting themselves in the scramble. It was certainly not a game or funny to them - but their mother would stand there and laugh her ass off. It was sick and sadistic.

There are lots of other examples....

I think it is only her healthy fear of me that has kept her from doing anything to my kids.