Oath of the Exorcists

For as long as there have been exorcists, there has been a code of honor and conduct: the oath of the exorcist. It's their code of chivalry that keeps them from falling off the deep end. Those who do are called Oathbreakers, which come in many forms including the horrid Soul Collectors being the worst of them.
Furthermore, are several interpretations and reimaginings over the years but there are some simple over-arching laws and generally accepted rules:

Do not leave without finishing a job Do not make company in beings of the spirit realm While exorcists may use all forms of magic, soul binding is forbidden