A Ghost Taming
The ghost taming art made things clearer to me regarding ghosts. A ghost was the spirit of a deceased person, that could not pass on to the next life, or whose rest had been disturbed. They were neither good nor evil, their behavior was based on the person they were in life and how they died.
Ghosts are also bound to an object or place, though this wouldn't be universal. Some powerful ghosts were entirely free. Ghosts often wear the clothes they die in. They bring things closely linked to them during their life into the afterlife.
Malevolent Ghosts were spirits that had done misdeeds in life. Either by killing, stealing, or sexual misconduct. Their soul bound to their resentment taking form. Any of those habits could cause someone to become a malevolent ghost after death.
This prisoner Ghost was under the sexual misconduct type. He was a pervert that liked boys. Resentment taking form? That must be a higher level of Soul Dao. A ghost was a soul fueled by resentment.