Dividing people into good and evil is a heresy and people get closest to evil when they are convinced that they are good (Lenin and Hitler were convinced of their goodness). The truth in this matter was well expressed by Alexander Solzhenitsyn in Gulag Archipelago :
Somehow that sounds unlikely. Greatness, perhaps. Superiority, perhaps. I doubt either of them considered themselves "good", and if they had, it would be irrelevant.
I'm just quibbling, perhaps, but bear with me. If a profoundly evil man considered himself good, it would not mean that anyone who considers himself good must therefore be profoundly evil.
Solzhenitsyn's line "... the line separating good and evil passes... right through every human heart, and through all human hearts " is profound and true. The struggle is never finished, the fight is waiting for us every morning when we get up. And the enemy is not only outside of us, he sometimes occupies territory within our own hearts.
Despite this, and perhaps because of this, it is not only possible to discern good and evil, it is incumbent upon us that we do.
God made us, and by his spirit and by right reason we are capable of recognizing evil, and rejecting it, and working against it, just as we are capable of recognizing the good, and working toward it. If we weren't, Solzhenitsyn's own insight would have been itself impossible.
Excellent, that's hitting the nail on the head.