If we want to kill sin, we must aim at the right target. That target is not merely bad behavior but the sinful desires of the heart that produce the behavior. Mortifying sin will certain bring about changes in what we say and do, but we need more than external reformation. Many people change their behavior without changing their heart to any significant degree. But Jesus is concerned about the root and motivation of sinful behavior-our drives and desires-not simply the behavior itself...The only way to kill sin is to mortify the roots of sin in the motives, desires, and drives of the heart. But to detect these desires we have to look comprehensively at our lives. You can't measure holiness by simply taking one or two slices of behavioral patterns...mortifying sin is not merely changing behavior, but rather addressing sinful desires in the heart.
Brian G Hedges, Licensed to Kill: A Field Manual for Mortifying Sin