One of a Christian's primary defenses against Satan of course has to be a prior awareness of his modus operandi, particularly—I might say right in this context here—his evil desire to turn good into evil. Perhaps no cunning could be more devilish than to do such a thing. But right in this context, Paul is alluding that Satan can get a person through a spiritual quality that is good.
When you put that together with verse 11, what Paul is saying is that a godly sorrow unto repentance can actually give Satan the opportunity to turn this person's feelings about his sin into an abnormal self-pity, which will destroy that despairing person's relationship with the church and with God by turning that person into a bitter cynic. Satan is that clever.
It does not end there. In addition to that, he can turn the righteous indignation of those who were offended by the man's sin originally into a bitter self-righteousness if they do not forgive and forget and go on. So he gets people going and coming unless they are aware that he is able to turn something that is good into a ploy by which he destroys that person's relationship with God and with the church.
