What does "go nasty" mean? Is it a common expression?
Here to "go all nasty" just refers to Finn's arm going out of control and injuring Susan. To say that something has "gone nasty" is not a common expression. However, the word "nasty" itself, which means that something is "unpleasant" or "disgusting", is a common word to use. Ex: I forgot to put sugar in the cake and it is nasty. Ex: Rats are nasty.