A butcher who killed animals for a living had a change of heart and became a lay Buddhist. He expressed great repentance for the killings and vowed never to do it again. Once these two conditions are satisfied, karma derived from the killing will become mutable karma which may or may not result in any karmic fruit. If the repentance could go deeper, it would even be possible that the butcher might not need to bear any consequence at all.

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