To be theologically fair, mankind was corrupted because Satan deceived Adam and Eve, biblically speaking. Once that happened, every human being since has been born sinful and corrupted.Or just as likely, borrowed it from earlier flood myths that existed in Mesopotamia at the time.
Even if you buy the story of a giant boat that housed two of every species on earth and their feed, (apologists like to say the word "kind" to make the number a little smaller-which of course would beg an entire new set of evolutionary questions) how we got to that point sounds a lot more like invented mythology than the workings of a divine omniscient and omnipotent being. The book of Genesis skips the goriest details, but you can find them relatively easily from other texts-the story of the flood has to do with Angels being assigned to earth, being overcome with lust for earth women (after all, spiritual, a-physical beings are all about the lust) breeding with them, and then spawning a race of Giants who behave very, very badly. The book of Genesis resumes at the story of the giants without explaining how they got here. The Giants corrupt mankind, except for Noah and his family. Yahweh looks down with disgust and decides to wipe out the whole darn thing and start from scratch.
This of course begs an entirely new set of questions, the chief one for me being how does an omniscient intelligence regret anything? This story, and dozens of others in the Old Testament, sounds like something a mortal man with superpowers would do, as opposed to a loving, omnipotent, omniscient, spiritual entity.
Whatever happened with the angels (who in reality were fallen angels/demons) procreating with women on earth (if that is what really happened, as the Bible does not directly say if it was fallen angels/demons that procreated with women, or perhaps demons possessing human men who then procreated with women), mankind was corrupted before then.
Also, just wanted to speak briefly about God's "regret." When we speak about God having regret, it is not synonymous with human regret. They mean two separate things. With God, regret incorporates the thought of compassionate grief and an action taken. God was not showing weakness, admitting an error, or regretting a mistake. Rather, He was expressing His need to take specific, drastic action to counteract the wickedness of mankind.
Interesting topics, for sure.