Rashi on Genesis 6:7/רש"י על בראשית ו׳:ז׳

1ויאמר ה' אמחה את האדם AND GOD SAID I WILL BLOT OUT THE MAN — He is dust and I shall bring water upon him and blot him out: that is why this root מחה is used (it expresses the idea of sponging out by means of wet or dampness) (Genesis Rabbah 28:2).

2מאדם ועד בהמה — FROM MAN, TO BEAST — These also corrupted their way (Genesis Rabbah 28:8). Another explanation is: All things were created on man’s account; when he ceases to be, what need of them? (Sanhedrin 108a)

3כי נחמתי כי עשיתים means for I have thought out what to do because I have made them. (Cf. this with Rashi’s second explanation of וינחם 5:6).