Poemen was once asked to comment on the phrase, “Do not render evil for evil” (1 Thessalonians 5:15).
Poemen spoke of the progression of evil in four stages: beginning in the heart, then in the face, then in words, and finally in deeds. He told the brother who asked that the place to defeat evil was in the heart (i.e. purity of heart). But if that was not possible to chase it through the other two stages (face and words) attempting to stop it before it reached the stage of deeds.
A victory anywhere along the way is still a victory, but once the deed is done, it cannot be undone.