John Philipps illustrates the conflict of the two natures in every believer that Paul discussed in Romans 7 and 8:
Certain types of criminals were executed by the Romans with special brutality. Sometimes if the man had committed a murder, he was bound hand to hand, face to face with the corpse of his victim and then thrown out into the heat of the Mediterranean sun. As the corpse decayed, it ate death into the living man and became to him, in the strictest literal sense, “a body of death” (Romans 7:24). Paul sees the carnal believer thus bound to the old nature and truly a wretched man (Exploring Romans, page 119).
Suppose a biologist were to perform an experiment by grafting at a given stage of development a butterfly to a spider and do so in such a way that the two creatures were fused into one and thus grew to maturity. What a clash of instincts there would be in a monstrosity like this. One part of the creature’s nature would long for the clear vault of heaven, while the other part would crave a web in a dark corner and a diet of blood. What could be done with such a creature? Nothing, except put to death. There is a sense in which, in the Garden of Eden, Satan performed just such diabolical surgery on the human race. Part of his own personality, so to speak, was grafted onto the human personality and the product of this union is the “flesh.” There is only one thing God can do to the flesh and that is to put it to death (page 120).
By the power of the Holy Spirit who indwells us believers we can put to death the influence of our sinful nature (Romans 8:13). MacArthur quotes the Scottish theologian David Brown, “If you don’t kill sin, sin will kill you” (The MacArthur NT Commentary, Romans 1-8, page 423).