Rude, and oblivious humans who speak before they think about what they are about to say. They are ciritical of the cooking made by their son's wife, or critical of the income earned by their daughter's husband.
If there are step children involved, they usually remain friends with their children's exes, and spoil the step kids to the point that the little bastards become spoiled brats who are rude to the new step parent and destroy their property.
The worst undeserved punishment life has for innocent people with good intentions.
NEW WIFE:(after being nice to mother in law) Wait, you guys are not even together anymore, wtf?
HUSBAND: She is concerned about her grand daughter.
NEW WIFE: Was she concerned when your ex was cheating on you and got pregnant by the other guy? I HATE MY Monster In-law.
*looking straight into her sons eyes with her hand on his hand, and his girlfriend sitting across from them by the table
" i cant deal with this anymore"- monster in law