I think we may be confusing evil with sin. The generalized concept of "evil" is something any human can know about and find repulsive. That's things like murder, theft, etc. The problem is that God is not only concerned with evil, but also with sin which goes much deeper. Every angry thought, every random judgement about that lady's funky dress at the mall, every little "white" lie we tell to make people feel better about things... They're all like getting a little speck of something in our eye, like we are all spiritually blind. And they all lead to our separation from God.
Evil tends to be things that happen on the surface, such as the actions we see outlined as against the Lord in the ten commandments. The actually sinful things, though, go much deeper. Consider when Jesus is speaking about Adultery in our hearts, telling us we need to love our enemies, etc.
A decent person can follow principles which will, largely and on the whole, prevent him or her from evil. However, the very nature of sin--which can often be invisible to the human eye without the guidance of the Holy Spirit to help us turn from it and Jesus' sacrifice to cleanse us of what we did prior to accepting him as Lord / what we mess up and do after that happens, is beyond the realm of any human to remain clean from no matter how hard we try. It is inevitable that we will mess up, messing up because of sin separates us from God, and only by accepting Jesus and what he did for us can that be set right.
I hope that helps paint a clearer picture, gentlemen.
This sub-branch of the discussion has been quite interesting.