Genèse 6:12

And the Lord beheld the earth; and, lo, it was corrupt; for all flesh had every one corrupted his way upon the earth.

FOR ALL FLESH HAD CORRUPTED THEIR WAY. If we were to explain all flesh in its usual sense and thus say that even cattle, beasts and fowl corrupted their way by consorting with other species, as Rashi has explained, we must then say that the expression, for the earth is filled with violence through them, does not mean “because of all of them” but only because of some of them [since “violence” does not apply to beasts and cattle], and Scripture tells of the punishment of man alone [even though all flesh corrupted their way]. Or we may say that the cattle, beasts, and fowl also did not follow their natural instincts, and all cattle seized prey and all fowl became birds of prey; thus they too committed violence.
By way of the simple meaning of Scripture, all flesh means “all men.” Further on Scripture says explicitly: all flesh wherein is the breath of life; and of every living thing of all flesh,, 6:19. meaning all living bodies. But here it says all flesh, meaning all people. Similarly [we find the verses]: All flesh shall come to worship before Me, [meaning “all people”]; Or when the flesh hath in the skin thereof, [where again reference is only to people].

והנה נשחתה, without external influences, as a natural consequence, not due to man’s ecologically criminal behaviour. Although on the surface, there appeared to be no connection, man’s corruption resulted in the corruption of his habitat. The language used here by the Torah is comparable to Isaiah 47,2 וטחני קמח, “and grind meal.” One does not grind flour, but one grinds grain into flour. The prophet spoke about the end product. The Torah also spoke about the end product of man’s corruption bringing in its wake, though invisibly, the destruction of the earth’s crust.

וירא, a continuation of the words לפני האלוקים in the last verse.
כי השחית, the same as והנה נשחתה, i.e. a passive not a transitive hiphil.
כל בשר, all of mankind. We find the same expression in Psalms 145,21 ויברך כל בשר, as referring not to all that lives, but to all of mankind. The same expression also appears in this sense in Isaiah 66,23 יבא כל בשר, “all of mankind will come, etc.”

את דרכו, its norms as established at the time of creation. Just as man cleaving to his wife (only) is described as a norm in Genesis 2,24, so there were other norms all of which were violated by the generation of the deluge. In retribution, G’d now violates the norms that applied to the conditions prevailing on our planet.