Proctor has committed adultery with Abigail and Francis Nurse does not believe it when Proctor admits this. When Proctor says that he wishes Francis had some evil in him so that he might understand, he is essentially saying that only someone who has committed an immoral act can know the feeling that follows - the shame, the guilt, the desire to hide one's crime and to recompense for it.
Proctor is also saying that he believes Francis Nurse is a good person who has no intimate knowledge of moral transgression and no personal history of crossing moral lines and wishing he could get back on the right side. Like John Proctor, Francis Nurse has a good reputation.
Nurse is a hard-working, honest member of the community...
However, Proctor is now admitting to his immoral actions and seeking to come clean with the community so that he might save his wife. Francis cannot believe or understand Proctor's claims, he cannot relate to Proctor's situation, because he has lived too good a life. The innocent man cannot relate to the repentant man.