I'm not going to tell someone that his fiance cheated on him with me
Why wouldn't you tell him about his cheating fiance?
I'd rather watch him suffer.
It is wrong to let someone suffer after doing wrong to them. You should tell him what happened.
What if I don't care?
How would you feel if you were cheated on? I don't think that would sit well with you, which is why you should let the man know.