We all make mistakes. From my own personal experience, when I do good for someone, I get the reward. On the other hand, if i've ever done someone wrong, I got what I deserved.
Yes, I do believe in "what goes around comes around." The concept is supported by many religions. In the Quran, we are told that we shall see the results of every atom of good and evil that we do. Also, I've read that it's better to get punished in this world than in the next world.
And I've even heard that karma is ***** because when it comes back to hit you, it tends to smack you 10 times harder. Some people believe that it comes back to bit you 3 times harder.
I think Islam allows us to exact revenge that is equal to the injustice done to us. If we exceed that, then we become transgressors. But it's much better to forgive.
Sometimes I do wonder why is it that bad people who treat others like **** are still thriving and living happily and experiencing success.....whereas the ones they are hurt are still struggling. But a friend of mine once said that such people are really very miserable on the inside. Because truly happy people don't treat others like crap. Only those that are miserable and bitter treat others badly. And it makes sense. Eventually, sooner or later, your *ty inner attitude is going to attract *ty situations!