Karma is thought to be very much like the Biblical "...an eye for an eye, a tooth for a tooth...", or the more poignant, "...sow what you reap..." In physics there is a concept that supports the view that "every action has an opposite and equal reaction."
Karma is thought to be the process by which energy returns to the individual in the same or similar form expressed by that individual. If the individual expresses violent energy, for example, that violent energy will return to that individual perhaps in this life, perhaps in the next.
The Beatles song, "Instant Karma" relates it this way:
Instant Karma's gonna get you,
Gonna look you right in the face,
Better get yourself together darlin',
Join the human race
These interpretations assume that there is some moderator or some moderating force in the universe that reflects the expressed energy back toward the expressor.
For a moment let's assume that there is no moderator, no god as deity, or universal entity, or process that keeps track of how an individual behaves. What do we have then? How then would this process operate in each individual life? In fact, the Buddhist philosophy from which the concept of Karma originates, assumes no entity as god, or existent deity.
It is also possible for those who read and study the Bible to assume no deity and perhaps better understand the sowing and reaping, and "eye for an eye" concepts.
The truth of Karma is that it is related to the individual's perception. And that perception is related to the way that individual behaves.
In the spiritual concept of emptiness there are no violent expressions, no loving expressions, there is only expression - all is empty of meaning until we impose a meaning on it.
When one behaves in what they perceive as a violent manner, their perception of the outside world follows and becomes violent as well. That sheds new light on the understanding of sowing and reaping. If one is harsh and brutal toward others they will interpret behavior toward them to be harsh and brutal as well.
There is an ancient Sufi story that illustrates this. An old man sits daily and greets travelers on their way to a nearby city. One day a traveler stops and asks him, "What are folks like in the city ahead?". The old man responds with a question, "What are folks like in your own city?" The traveler responds, "They are mean spirited, talk ill of others, and are very selfish." The old man then answers the original question, "Well you will find the people in this city to be much the same way."
The next day a second traveler asks a similar question and receives the same question of the old man. This time the traveler states, "The folks in my city are kind and honest, selfless and loving." The old man answers him, "Well you will find the people in this city to be much the same way."