Commentary on the Sūtra of the Collection of the Father and Son (171)

We have created a great deal of unwholesome karma over our countless lifetimes. After enlightenment and attaining the fruits, we no longer need to go to the three evil destinies to undergo retribution; it is enough to undergo suffering among humans. In future lives, we may either be reborn in the heavens to cultivate, or cultivate among humans, and will not again fall into the three evil destinies. Unless bodhisattvas have the ability and the power of vows to descend into the three evil destinies to deliver those sentient beings or disciples whose karmic affinities have matured. For example, if a certain disciple has very deep roots of goodness but occasionally, through carelessness, creates karma leading to the three evil destinies and falls into them, a bodhisattva who has the ability can follow him into the three evil destinies to deliver him—this is the bodhisattva's aspiration. However, bodhisattvas descend into the three evil destinies not because of unwholesome karma, but because of the power of their vows; their suffering is far lighter than that of the sentient beings who go there due to the power of karma, because bodhisattvas have wholesome vows.