The verb שִׁחֵת {corrupt} is very common in Holy Scripture, and it is striking. Moses also employs it in Deut. 31:29: “I know that after my death you will surely act corruptly; and turn aside from the way”; and David, in Ps. 14:3: “All are corrupt and have become detestable.” Furthermore, both passages are really speaking of sins against the First Table; that is, they charge those who are saintliest in appearance with false worship of God and with false doctrine. It is impossible for an ungodly life not to follow in the wake of false doctrine.
When Moses states that the earth was corrupt in the sight of God, he clearly points out the contrast, namely, that the hypocrites and tyrants were of the opinion that what Noah taught and did was wrong, but that they taught and did everything in the saintliest manner. But, says Moses, the opposite was true. The earth, that is, the entire world, or all human beings, was corrupt, namely, so far as the First Table is concerned, they did not have the true Word or the true worship. This distinction in regard to the First and Second Table is very much to my liking and has undoubtedly been pointed out by the Holy Spirit. (Luther’s Works, v.2 p.158)