What kind of person commits incest?

No single predictor. Incest comes in many forms and has many causes. In some societies, it has been acceptable behavior. Our society has a strong prohibition against--and fascination with--it, so something has to overcome that taboo. One common factor is that one or both parents were themselves sexually abused by a relative as a child. Often, it like sexual abuse broadly, is as much about power as it is about sex.