Only God knows how evil the world was. In all probability it was no better or worse than it is today, but to day we are living under the promise that God will not flood the earth again. Noah was spared because he was righteous before God, but he had to take steps to secure his salvation by building the ark. The ark was big, so he couldn't do it in secret.
What we have is the sovereignty and prerogative of God. It didn't matter who you were. descendant of Seth, descendant of Cain, or one of the Nephilim, bye bye to you. You were either evil, or subject to evil influences. The significant statement is that Noah found grace before the Lord. He walked with God. Judgement is inevitable - making friends with the judge is highly recommended.
I suppose in a sense all judgement is cruel. Here it was not intended to punish but to eliminate, absolutely and completely. The story is not about the flood it is about the ark. Noah found grace he was in the ark. The ark is a picture of Christ. Everyone inside the ark was saved. Maybe they had no particular merit other than they had found grace with God. Similarly, those who are in Christ, through faith in him have found grace with God.
We don't deserve salvation, it is a gift of God, because of His grace, not because of anything in us.