Musar zu Bereschit 18:23

ויגש אברהם . The Midrash Ne'elam of the Zohar on this verse (Sullam edition page 60) quotes Rabbi as saying that it is a מצוה for a person to pray that sinners become penitent so that they will not wind up in purgatory. We know this because it says in Psalms 35,13: ואני בחלותם לבושי שק, "Yet when they were sick, my dress was sackcloth." [David prayed when sinners suffered afflictions. Ed.] Rabbi also says that it is forbidden to pray that the sinners should be wiped out, for if G–d had wiped out Terach while he was still an idol-worshiper, our patriarch Abraham would never have seen the light of day, there would have been no Jewish people, neither would there have been a David. Torah would not have been given to mankind, and all the righteous and pious people would never have existed.