Well, I could say that's what happens when women enter the picture but I'd better not. :O)
I would suggest you answered your own question. The only logical and rational explanation is that this was all part of the plan of God. And it was to simply show us how we are not as "magnificent" as we think we are. In fact, we are bent towards hell and destruction from day one as evidence when the first law was given not to eat of the forbidden fruit. The very thing that we shouldn't do, we do. And its been that way ever since. Everything that God does, every law that He has given, has been for our benefit and our care. And yet we continuously disobey those laws and commands. To help us over this He has given us His grace which we still refuse if it wasn't for Him opening our eyes and ears.
As far as Noah goes, you're underestimating the wickedness and corruption of man. God's wrath exists because it is the only way in our wickedness we will respond to bring us back to Him. If you have any doubts, look at what happens when we have a catastrophe. People flock to the churches all the while questioning how a loving God could do such a thing. (Yet, off they go the next day into a life of sin.) For God to have destroyed the whole earth except Noah and his family simply shows how far we can go as a people.
Christ did not come yet because it wasn't the "fullness of time". It's all part of a plan.
If I might add to that, I think we look at these moments where God acts against our wickedness and we only think about the wrath, but we forget the mercy. Noah is a perfect example, the world had become so wicked that God cleansed it. However, He saved a remnant. IOW, God is just but is also merciful where none is deserved.