“And the L-RD saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually.”
– Genesis 6:5, JPS 1917 Tanach
“Ten generations from Adam to Noah, in order to make known what long-suffering is His; for all those generations kept on provoking Him, until He brought upon them the waters of the flood.”
– Pirkei Avos 5:2, sefaria.org
“In G-d’s long-suffering we can learn a lesson of patience and forgiveness. Even though in the end G-d did decide to destroy the world, He did not do so immediately, but gave the world a chance to repent“ (English Explanation of Mishnah; sefaria.org). Therefore, we can see that G-d is not only just; he is also merciful. It is only fair to give others a second chance in life, as G-d did with the generation of the Flood. On a personal level, although we do not know what the person being forgiven will do in the future, it is up to us to attempt to amend the situation. And, moreover, to caution others against aveiros (sins), and expound upon the importance of teshuvah (repentance), is humane. We can not foresee whether others will do teshuvah or not; yet, we must give others the opportunity to mend their ways.
The Tree of Life 