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 the doings of defiled mortals. 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 not stable— being good now does not mean staying good forever. If we were to have the powers of divination, we would be able to see all of our negative karma being stored in the alaya consciousness. Without repentance, the ensuing retributions will surely take place. Then it will exactly match the nihilists’ view, i.e., causality does not exist, such that one may lead a virtuous life but still drop to the lower realm after death. That would be a dangerous view for us Buddhists. Thus, we must repent all our negative karma as all of them can be purified through true penitence.

- Quote from The Right View, "On Cause and Effect"