turn nasty
To become unpleasant or troublesome. I didn't know they had a problem with each other until things turned nasty and resulted in a screaming match at my dinner party.
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.