One thing I've wondered about is this: God destroyed (most of) the people in the world to rid it of evil, right? Since Noah and his family were "perfect", then how did sin re-infest the world?
When you read through Genesis, you will find certain places that show Satan's attack upon the Seed of the Woman, i.e., the Seed that Jesus Christ would be born through. Satan tried to destroy that Seed which points to his wanting to prevent Christ's birth in the flesh.
By Satan's angels leaving their own habitation and going after strange flesh (Jude 1), the Nephilim (giants) were used to try and taint the Seed of the Woman, which if you think about it would have meant a lot of success in preventing Christ's later birth through woman. This was the real reason for the world-wide flood of Noah's day, and also God telling Israel to wipe out the seven specific nations of the land of Canaan, because that's where another irruption of the giants had happened after the flood, as they had evidently corrupted the Canaanites also into doing wickedness.
Gen.6 mentions those giants were the men "of renown". That means men of the legend or myth, mythologies. Per many of the ancient writings of mythology, the giants apparently had superhuman attributes, not just physically but spiritually also. People on earth began worshping them as gods, knocking God's Salvation Plan out of balance. So God destroyed them off the earth.
In some OT Scripture, the Hebrew word for Rephaim, a name for the giants, was instead translated to "dead" or "deceased" in the English. This is one of them:
Isa 26:14
14 They are dead, they shall not live; they are deceased, they shall not rise: therefore hast Thou visited and destroyed them, and made all their memory to perish.
KJV
Do you know what that "they shall not rise" is in reference to? That's about the idea of resurrection. The spirits of those dead giants will not be resurrected from the dead. That's another proof that the 'giants' (Nephilim) of Gen.6 was not simply about bad men turned to bullying, but an actual race of hybrid offspring from the fallen angels that were outside of God's Plan for this present world.