They weren't the only reason for the flow...they earth at that point was bad all the time. The flood did get rid of the Nephalom(sp) but not the spirit that made them. They went back into heaven. Yes heaven had evil spirits at that time which is the point of there being a 'New Heavens'. This is the point. The bad spirit resided in heaven during this time and for a while. Satan said that man would curse God if faced with temptation and he said that while residing in heaven. When one sins, they die. The angels aren't immortal but they don't grow old and die like we do.