Everyone has a good side and a hidden bad side regardless of how amazing of a person you are. For some a trigger like being cheated on can instantly bring out that monster within, and that's when they start doing things out of their character.
We have seen stories of how people spray-painted their ex's car with the word 'cheater' in all caps and there's even others who sabotaged their cheating partner's career completely. Some might also choose more discreet ways like cheating back as a way of teaching their partner a hard lesson.