Musar על סנהדרין 207:23

Repentance is also good for the bad, that you [should] not say that repentance is only good for the holy part in man - but rather it also sweetens the evil part in him, similar to this [Higher] trait: Know that Kain was bad and was from the snake, but it was stated to him (Genesis 4:7), "Is it not that if you do good, it will be lifted" - do not think that because you are from the side of evil, that you have no reparation; that is a lie! Is it not that "if you do good" and root yourself in the secret of repentance, "it will be lifted," [such] that you will withdraw to there in the secret of the good rooted there - as all the Highest Bitterness has a sweet root and can enter through its root to better itself. And so these same actions improve a person, and his 'volitional sins are made for him [to be] like merits." As behold, those actions that he did were prosecuting from the 'left side.' [If] he repented with complete repentance, behold, he brings these actions in and roots them to the Above. And all of these prosecutors do not become nullified, but rather improve themselves and root themselves in holiness, similar to the [possible] betterment of Kain. And behold, if Kain had repented and been repaired; behold, the volitional sin of Adam (literally the first man) through which he fathered Kain - who is the nest (kina) of impurity - would have been considered a merit, through the secret of 'a child gives merit to the father' (Sanhedrin 104a). However, he did not want to repent. And hence, all the 'side of the left' is drawn from there. But all of its branches are destined to be sweetened, and they will repent and be sweetened. And this is exactly from the reason that we explained - that a person takes the secret of evil to his own roots and sweetens it and brings it into the good. Hence, a person purifies his evil impulse and brings it into the good, and it becomes rooted in holiness Above.

* So too, for all the ways of kindliness, such as "a son brings merit to the father" (Sanhedrin 104a), or "part of a life like the whole of a life" (Kohelet Raba 7:25) mentioned by the Sages which are aspects of kindness to account a small amount as a large amount. But these do not really conflict with or contradict the attribute of justice. For there is already a proper reason to consider them [as justice].