Is revenge an evil?

No I believe revenge is the meting of justice if in proportion to the affliction.

I don't know what I'd do with a cheating partner, I would probably do nothing at all (apart from break up, obviously).

Have you experienced this? If yes, I consider you an extremely tolerant person. If no, I think you'd have to go through it to consider what your actions would be.
I know mine would be deadly.

Experienced being cheated on? No.

My (expected) reaction is nothing to do with being tolerant, it's just that I wouldn't want to be near the other person, or be involved with them in any way.

But the question is moot, I won't be going through it