get nasty

To become unpleasant or troublesome. I didn't know they had a problem with each other until things got nasty and resulted in a screaming match at my dinner party. OK, I know you're mad, but there's no need to get nasty. Can't we discuss this like two civilized adults? If he cheated on her, then their divorce proceedings will definitely get nasty.

get/turn ˈnasty

1 become threatening and violent: You’d better do what he says or he’ll turn nasty.
2 become bad or unpleasant: It looks as though the weather is going to turn nasty again.