Life= Karma Or Luck

Have you ever wondered about the statement “Life comes a full circle”. There are so many people who cheat, steal, betray, lie etc basically do things which aren’t considered virtuous, but they do not seem to be punished always. Sometimes, they are never punished at all. In fact, my observation has been that it has been the other way round with these people. They seem to be happy always & enjoy life to the fullest. Luck seems to be always in their favor & I really wonder “does every dog really have its day?” I still haven’t found the answer to these questions that haunt me so often.
My mom often says, “Do good and it will come back to you”.
Well, I am still waiting. I read somewhere that “what goes around, comes around”. I searched the Internet for definitions of karma & tried to understand the principle on which it functions. "The law of karma, it read, refers to the law of action and reaction, also called the law of cause and effect. This law states that what we sow we shall reap in this or future lives. Benevolent actions (punyakarma or sukarma) will bring loving reactions. Selfish, hateful acts (papakarma or kukarma) will bring suffering. Every action that we perform in life, every word we speak, even every thought that we think, has its reaction. I am still not convinced. I feel that many people get away with wrong doings of various kinds very easily. So then, should we call it destiny then?

For example, an individual is really nasty to me, so I feel the impulse to retaliate and be nasty to him. But the law of karma & more importantly a lot of my elders say that if I follow that track I will create a new unseemly karma to face in the future. There is no need for me to be the instrument to return a karmic reaction to someone else. I think it’s better to let the law of karma take its own course without my intervention, which will generally happen through some other person with less self-control who does not understand this law of life.’
But then that person is always happy. Irrespective of the fact that I do something to retaliate or not!
How do you explain that?