Tafsir of Ash-Shu`ara' 26:157

It is narrated that Musṭaʿ forced her into a narrow mountain pass, shot her with an arrow, and struck her leg, causing her to fall. Then Qudār struck her. It is also narrated that the one who hamstrung her said, "I will not hamstring her until you are all satisfied." They would enter upon the women in their private quarters and ask, "Are you satisfied?" and they would say, "Yes." They did the same with their children.

If you ask: Why did the punishment seize them when they were regretful?

I say: Their regret was not the regret of those who repent; rather, it was the regret of those who fear that they will be punished for the hamstringing immediately. It is like someone who holds a corrupt opinion on a matter, acts upon it, and then regrets it and feels anguish, like the regret of al-Kasaʿī. Or, they regretted it with the regret of those who repent, but at a time when repentance is not accepted—that is, upon witnessing the punishment. God Almighty said: "And repentance is not for those who [continue to] do evil deeds..." (Al-Nisāʾ: 18).

It has also been said that their regret was over leaving their offspring behind, but this is far-fetched. The "al" (the) in "the punishment" refers to the punishment of a Great Day.