Genesis 6:12

"God saw how corrupt the earth had become, for all the people on earth had corrupted their ways." Genesis 6:12

It says the people "corrupted their ways." This isn't something that happened to them, but a decision the people made about their own lifestyles.  They had a decision and instead of choosing the righteous path they chose evil.  I'm sure they didn't wake up one morning and think, "I'm going to be evil now," but just one little decision at a time.  "One little white lie won't hurt anyone."  "I need it more than he does."  "But it feels good."  One step became twenty became a hundred and before they knew it they were enemies with God.  What once was clearly wrong became normal and if it's okay for you than it's okay for me too.  The devil can only lead us away from God if we let him.