God sees sin, even when the world calls it normal.
In Noah's day, wickedness was not hidden. It filled the whole earth, and it started in the heart.
Genesis 6:5 (KJV): And GOD saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually.
The world did not see itself as corrupt. God did.
Genesis 6:12 (KJV): And God looked upon the earth, and, behold, it was corrupt; for all flesh had corrupted his way upon the earth.
Sin does not just break a rule. It grieves the heart of God.
Genesis 6:6 (KJV): And it repented the LORD that he had made man on the earth, and it grieved him at his heart.
God gave them time, but His Spirit would not always strive with man.
Genesis 6:3 (KJV): And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.