Although we have not committed killing or stealing in this life and, being Buddhist practitioners, we often chant mantras, meditate and liberate animals, these virtuous actions are still those of a being with defilements. Once strong anger arises in our minds, all our virtuous karma so far accumulated will be destroyed instantly if the merit has not been dedicated. Besides, the roots of virtue of ordinary people are unstable—being good now does not mean staying good forever. If we were to gain the power of clairvoyance, we would be able to see all of our negative karma that is being stored in the alaya consciousness. Without repentance, the ensuing retributions will surely take place. That would exactly match the nihilists’ view—that causality does not exist—in the sense that one may lead a virtuous life but still fall to the lower realms after death. That would be a dangerous view for us Buddhists. Thus, we must repent for all our negative karma since all of it can be purified through true remorse.
~ Luminous Wisdom Book Series 1 : On Cause and Effect