It sounds like your ex is a person who got seriously damaged somewhere along the line. Probably something was wrong in the dynamics of the family she came from. Nobody gets that twisted all by themselves. She had help.

It's good that your kids are with you in a stable environment where they feel safe and loved. They deserve that. Hopefully, they can come to see their mother as a troubled person. They should not be put in situations where they are vulnerable to being maltreated by her. I wouldn't force them to hang out with her. But feeling contempt toward her should be discouraged. For all we know, how she is may be all that she's capable of being. That doesn't mean she gets a right to make trouble for you and the kids. Boundaries have to be set and maintained. Life will impose a cost on her for any bad choices she has made or will make. If she stays remote from involvement with her children, that may actually be in their best interests.