Emotional pain is often caused when a partner decides they can play the field, and the person they hurt the most may not always be the one who walked away from them. That person may have their own sense of morality that holds them to the standard of being faithful, and they might just find someone better once the relationship is over. Running into them a year or so later could show that the best revenge is their happiness with someone else, and the player has no one else to blame for losing a great partner.
Playing the field and cheating on a partner is usually not about the other person. They will be affected by what the player does, but their own values may never be compromised. The person willing to go behind the other person's back and date outside their committed relationship is usually attracted to the thrill of getting away with something illicit. This type of behaviour points to an issue in their life, so blaming the person they cheat on is nothing more than another issue of their own.
Walking away from a partner who has been caught cheating repeatedly can be a time when pain overwhelms a person unable to comprehend why it happened. Their cheating partner may have many reasons why they were not worth being faithful to, and it can take time to realize it had nothing to do with them or the relationship. Moving on might be difficult, but finding someone better is almost always the outcome.
The person who walked away from a cheater might be dealt a serious blow at first, but there are plenty of other people looking to share a faithful relationship. Finding one of them for a truly great relationship is a definite possibility, and being happy together could be the best revenge of all as they run into the former partner still living a life that will never be truly satisfying.