This question asks for an idiom for someone getting what they deserve. But what about the feeling one gets when someone else gets "what they deserve"? Is there a specific word or expression for the good feeling you get when justice is served?
Albeit only negative...
Schadenfreude — Dictionary.com
noun 1. satisfaction or pleasure felt at someone else's misfortune.
Some I would use:
fulfilment of one's wishes, expectations, or needs, or the pleasure derived from this.
a great victory or achievement.
pleasure, especially when gained from the satisfaction of a desire.
Although it would no longer be a single word, you could prefix these with something like Karmic to really enforce the reason behind the feeling.