This soul is eternal, it has been travelling through thousands of life times. Finally it has reached to the stage it is now, with this human body. The consciousness has evolved up to the level where it got this life time of being a human. Now, this soul, while going through it's journey collects many impressions about other souls. Similarly, other souls also collects impressions about this soul. These two types of collection of impressions collectively is called KARMAS.
This life is a part of process of evolution for the soul. During this process of evolution, the karmas that help you uplifting your soul, helps you making this process of evolution faster, are your merits (Sanskrit terminology is punyas). However, the karmas that becomes hindrance on the way to evolutions, makes the process slower, are sins (Demerits - Sanskrit terminology is papas). When you hurt any one (knowingly or even unknowingly), they carry an impression about you where whenever they remember you, the emotions, the sensations of hatred, hurt triggers in them, which not only harms them but also being your karmas harm you as well, since they become hindrance in your process of evolution. n that's why in any religion in the world, the prime importance is given to NOT TO HURT ANYONE.