" The Lord saw that the wickedness of man was great on the face of the earth, and that every intention of the thoughts of his heart was only evil continually" - Genesis 6:5
It says in many places in the Bible that sin originates in the mind. As men we are just as capable of sinning with our thoughts as we are of sinning physically in some cases even more so because we forget that there is an ever present Watcher who knows our thoughts. In Christ we are called to hold our thoughts captive to the Word and love God with all our heart soul mind and strength bending every wayward idea to Christ's standard. In the time of Noah these men were constantly dwelling on evil causing their words and deeds to become increasingly evil to point where, "seeking to become wise they became fools exchanging the truth of God for a lie worshipping the creature rather than the Creator" (Rom 1: 22-23)
Because of man's fallen nature he desires evil; there is no good in him. As Christians we must not allow ourselves to be led astray by those who should be led to Christ. Our thoughts can be held captive by Christ or the World and we must fight to "work out our salvation with fear and, trembling for it is God who works in you, both to will and to work His good pleasure."
(Phil. 2: 12b-13)