To grasp the concept of Karma, you have to move out of dualistic thinking.
Good and evil do not exist in themselves. They are our subjective judgements.
We are spiritual beings having an experience in physicality; we are here to experience.
All things have a positive and negative (that does not mean good and bad! think of electricity if that helps... or two sides to every coin). In spiritual terms, they are One; opposites are one. Joy and sadness are two experiences of the same thing, as are pain and pleasure, birth and death; they are a movement, a process, a verb.

Just as a coin has two sides, but you can only look at one or the other at a time, even though you hold both in your hand.

So, using that analogy, if you pick up a coin, you have both heads and tails, and you being a soul here to learn and experience what you pick up, you shall look at one side of that coin, and eventually turn it over to see the other. Then you know the coin.
Here, once you have "picked up" a movement, or process, you experience one side of it, and will eventually experience the other side of it, in order to know it as a physical experience in it's entirety, or wholeness.

There is much misunderstanding in the western world about this concept because of the good -evil, moralistic thinking. It has nothing to do with punishment or reward, and the concept of a enlightened or older souls freeign itself from karma is a matter of having been through enough physical lives to have already learned, experienced, and known each side of every process- having already killed and been killed, been rich and poor, been joyful and sad, etc.

Once a souls has learned these experiences, it gets closer and closer to being able to live in a way which is more balanced, less dualistic, more moderate- the eye of the needle in which one takes paths neither of one extreme nor the other, but more as the spiritual reality, in which both are one.

Therefore, those with whom you have karma are neither your enemies, nor your punishers, nor debtors, they are your "soul friends" and you each are enabling each others experience in physicality.

The ultimate reality is that there is not actually a separation between I and Other, even that is an illusion created by the linear time and physical space of this plane of existance- so you cannot do anything to another that you yourself won't have done to you! because there is only One.