I understand where you are coming from. My son's father is abusive and yet I keep hoping he will come to his senses. He has not done anything as bad as your ex-wife (I don't think - although more and more is coming to light about him.) However, I don't like to deprive my son of his father and at the same time I know it is bad for my son to have to deal with him. It is a hard line to walk!!!!
Maybe what will help is the experience of a childhood friend. We were both about 14 years old and he kept saying how he wanted to meet his bio dad, how life would be better with him, etc. (His JW stepdad was also abusive to him and his mother.) Well, he was taken to Texas to meet him and his bio dad was drunk at the time and it crushed this poor boy so much that I don't think he ever really recovered.
I think the best thing for my son, even though it will be hard to go through, is when the courts finally proclaim that my ex is unstable enough to be denied any visitation. It will hurt me to see my child go through it, but it is not me that has been violent, abusive and mentally unbalanced. It is not my son's fault either, but something that he will have to deal with at some point.
Protect those kids, even though it is hard!!!