No one’s karmic history is a single account. It’s composed of the many different seeds planted in many places through the many different actions we’ve done in the past, each seed maturing at its own rate.

"When you look at people, you can’t see all the karmic seeds from their past actions. They may be experiencing the results of past bad actions, but you don’t know when those seeds will stop sprouting. Also, you have no idea what other seeds, whatever wonderful latent potentials, will sprout in their place.

There’s a saying in some Buddhist circles that if you want to see a person’s past actions, you look at his present condition; if you want to see his future condition, you look at his present actions. This principle, however, is based on a basic misperception: that we each have a single karmic account, and what we see in the present is the current running balance in each person’s account. Actually, no one’s karmic history is a single account. It’s composed of the many different seeds planted in many places through the many different actions we’ve done in the past, each seed maturing at its own rate. Some of these seeds have already sprouted and disappeared; some are sprouting now; some will sprout in the future. This means that a person’s present condition reflects only a small portion of his or her past actions. As for the other seeds, you can’t see them at all.

This reflection helps you when developing compassion, for it reminds you that you never know when the possibility to help somebody can have an effect. The seeds of the other person’s past bad actions may be flowering right now, but they could die at any time. You may happen to be the person who’s there to help when that person is ready to receive help."

~ Thanissaro Bhikkhu "Head & Heart Together: Bringing Wisdom to the Brahmavihāras"