Insecure about their relationships with their husbands. They try to make the ex look like a psycho bitch cuz that pleases their husbands, without thinking that if your husband talks about his ex like that, chances are he speaks of you the same way.
I also think trying to turn the kids against their parents is pretty f'ing low. If mom/dad is a lowlife, letting them know makes the child feel like they are the same peice of trash.
Kids should never know of the conflicts between parents, nor should they be used as pawns. Doing so damages them, so much!
If step parents were truly out for the best interest of the children (which they should be, being that they entered into a relationship with a person who already had kids), they would not let petty things like haircuts, pickup times, and child support be determining factors in their relationships with their SK's.
I have been on both ends, and know damn well all the shizznit I said about BM was out of anger.
I do realize that BM's often hate on the SM. But, they are hurt and/or angry. They are not insecure, b/c there is nothing to be insecure about. They no longer have an intimate relationship with BD. The SM's I am referring to are the ones who go out of their way to make every single mistake BM makes out to be horrible parenting. The ones who complain about arrears being spent on BM, without thinking that BM already spent her own money to supplement BD's when he wasn't paying. There are good SM's out there. However, if you badmouth your SO's ex for everything, chances are, you are not.
Edit No 2: Since so many people seem to have missed the point, here. I do not justify BM's being rude and mean to SM's. But, when BM got pregnant, she most likely did not know she would later have to deal with another woman in BD's life. It was just a card that fate dealt her. She got the short end, KWIM? SM, on the other hand, knew exactly what she was getting into when she started dating a man with children. She can't undo the kids or the fact that her SO's ex will always have a relationship with "her man." Which is when most resort to the insults and badmouthing.