Evil is what is produced by sin, or in other words it is the activity of sin. The believer overcomes the evil from sin because Christ has overcome the sin in the believer; thus our actions are not what overcomes, but is evidence of having overcome - through Christ’s atonement in our stead.
An example of evil from without is that of Romans 12:21 which instructs, “Be not overcome of evil, but overcome evil with good.” The evil in reference here are the wrongs which others may do unto you (vs 14, 17); in which if responded to in like manner (other than physical self-defense) places us to the allowance of being “conformed to this world” (v 2). An example of evil from within is that of Romans 8:13 which instructs the believer to, “mortify (not eliminate as by death but subdue as impair) the deeds of the body, through the Spirit” and for this I chose to use John Gill’s comment below.
“Through the Spirit do mortify the deeds of the body”; This is not to be understood of the mortification of the body itself, nor does it design any maceration or afflicting of it by any severities of life, nor of the destruction of the body of sin by Christ or of the being and principles of sin in the saints by the Spirit of Christ; which is contrary to Scripture and to the experience of the saints who find it alive in them and to their expectations whilst in this world.
Sin, as to its being and principle is so far from being destroyed that it seems rather to revive in the sense and apprehension of regenerated persons. But it is a mortification of the outward actings of sin in the conversation, called, "the deeds of the body". That is, concerning its outward course of life and signifies a subduing and weakening the vigor and power of sin in the lives and conversations of the saints, to which the grace and assistance of the Spirit are absolutely necessary.”
Material from W M Newell well explains that “A great many people think that while sinners have no power against sin, saints have; that is, that God gives the new creature strength in itself to overcome indwelling sin. But this is a fatal error. Many, many Christians today are struggling against sin, to “pray for divine help” and then “fight the good fight” against sin. So, they struggle manfully, but with what sorry success because their whole theory is in error. It is because they do not finish the verse “Fight the good fight . . . of faith.”
God has not given us, even in our new nature, power over sin. His plan is entirely different: He Himself becomes the power in us that overcomes sin. He does not delegate this power to us. He said, “All power is given unto Me,” but He Himself exercises it within us, in the person of the Holy Spirit, who has come to dwell in us for the very purpose of delivering us from the power of indwelling sin.”