As Christian counselors, pastors and people helpers we often have a hard time discerning between an evil heart and an ordinary sinner who messes up, who isn’t perfect, and full of weakness and sin. I think one of the reasons we don’t “see” evil is.
Above all else, guard your heart, for everything you do flows from it. What is in our hearts comes out through our words and actions. We must guard our hearts from evil so that we can abstain from doing evil.