15 As yet the punishments of the lower places have not come, as yet fire everlasting has not come: let him that is growing in God compare himself now with an ungodly man, a blind heart with an enlightened heart: compare ye two men, one seeing and one not seeing in the flesh. And what so great thing is vision of the flesh? Did Tobias by any means have fleshly eyes? His own son had, and he had not; and the way of life a blind man to one seeing did show. Therefore when you see that punishment, rejoice, because in it you are not.
Therefore says the Scripture, “The just man shall rejoice when he shall have seen vengeance”. Not that future punishment; for see what follows: “his hands he shall wash in the blood of the sinner.” What is this? Let your love attend. When man-slayers are smitten, ought anywise innocent men to go there and wash their hands? But what is, “in the blood of the sinner he shall wash his hands”? When a just man sees the punishment of a sinner, he grows himself; and the death of one is the life of another. For if spiritually blood runs from those that within are dead, do thou, seeing such vengeance, wash therein your hands; for the future more cleanly live. And how shall he wash his hands, if a just man he is? For what has he on his hands to be washed, if just he is? “But the just man of faith shall live.” Just men therefore he has called believers: and from the time that you have believed, at once you begin to be called just. For there has been made a remission of sins. Even if out of that remaining part of your life some sins are yours, which cannot but flow in, like water from the sea into the hold; nevertheless, because you have believed, when you shall have seen him that altogether is turned away from God to be slain in that blindness, there falling upon him that fire so that he see not the sun— then do thou that now through faith see Christ, in order that you may see in substance (because the just man lives of faith), observe the ungodly man dying, and purge yourself from sins. So you shall wash in a manner your hands in the blood of the sinner.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)