I think that part of the reason that they become so cruel and say such hateful things is because they need to justify their actions. They are doing something that they cant even approve of on when its true face value is exposed. Honestly: He is walking away from his spouse and child, to be with another woman! Has he ever approved of adultery? Im guessing that he has not. My H HAD impeccible morality where M and vows were concerned. They cant live with the true nature of what they are doing. Denial! So they make up terrible things to think about you, and they repeat them so much that they honestly beleive them, and the OP fuels it.
And then, one day, the OP is gone and they realize what they have done. And it will be crushing for him.
I guess I gave the wrong finger to the wrong man...