User blog comment:FairyRaven/Fairy Tail Chapter 368: The Third Seal, Review and Commentary/@comment-24039250-20140124222628/@comment-7418318-20140126010730

Or he learned that magic so he'd have an easier time to climb within Tartarus ranks, if they get promotions by killing their superiours in one-on-one combat for example.

Or he originally was a Demon Hunter but was caught and turned after days/weeks/month/years of torture or out of some personal reasons like Heartbreak, Revenge, Alcoholism, Realization that he was only used, Addiction to Power, Fear of his own mortality,...