According to Genesis 6:12, the pre-flood world was filled with violence. Concerning what God thinks of violence, Psalm 11 states:
...And the one who loves violence His soul hates. (Psalm 11:5)In the time before the flood, the book of Genesis mentions only a few specific violent crimes. Shortly after the fall, the most famous of these crimes was executed: Cain murdered the righteous Abel (see Genesis 4:8). Later, the book of Genesis also mentions that Lamech killed a man (see Genesis 4:23). However, these violent crimes were were just two among many other violent crimes committed in the pre-flood world. As the human population expanded and as the wickedness of man grew greater, violence also expanded until it filled the earth.
It would be reasonable to speculate that by the time of the flood, full-fledged wars were taking place; evil men fought evil men so that they could increase their evilness. In such wars, there would not have been a "good" side and a "bad" side. There would not have been one side fighting for justice and one side fighting for corruption. But there would have been two bad sides, competing against the other, in an effort to advance their lusts. Concerning the wicked, Psalm 7 states:
His mischief will return upon his own head,Man's selfish violence in the pre-flood world did not result in the formation of a utopia on earth. Instead, man's love of violence made him an enemy of God. The violence through which men sought to gain victory would be returned upon their own plates in a ferocious flood.
And his violence will descend upon his own plate. (Psalm 7:16)
The Pre-Flood World: Filled With Violence (Part 3) - Genesis 6:11-13
Noah: Blameless In His Time - Genesis 6:9-10
The Pre-Flood World Soon To Be Blotted Out - Genesis 6:7
Noah's Name: Its Meaning and Significance - Genesis 5:29
Cain and Abel: Two Routes - Genesis 4:1-2