Background
Sting grew up poor on Rodia in a disgraced clan of failed hunters. His family was deep in debit to the local crime syndicate. He was offered a chance to do some work for the crime syndicate to help pay off some of his clan's debit. To which he agreed. He quickly made a name for himself as a top notch enforcer and collector. Soon he was taking on the role of a lone shark and book maker, a role in which he excelled at given his past reputation as a gang land enforcer. This ensured that he rarely had to collect outstanding markers, but when he did he was sure to visit the mark personally and get the money by what ever means necessary.
With his growing reputation the syndicate moved him to the more fertile location of Nar Shaddaa where he took to his new life like a fish to water. He was able to increase the syndicate's revenue by 30% by quickly showing people that he was not a shylock to be skipped on.
Things were working out well until a Fel Draga, a Neimoidian crime boss that was jealous of Sting's budding fiefdom of crime, convinced Sting's marks that Fel's organization would protect them and renegotiate their payments if they refused to pay Sting.
As can be expected, this soon turned into all out warfare between the 2 factions. During the fighting, the daughter of, Gikk Nogun a Nikto minor crime lord was inadvertently killed when the fighting spilled over in to a market area. While Gikk does not blame Sting directly for his daughter's death, it is still something that he expects Sting to pay him back on.
With Sting coming out on the losing end of the fight, the syndicate withdrew their support and removed him from the organization. The only reason that he is still alive, is that they still may have a need for him in the future once things cool down with Gikk and Fel.
In the mean time Sting has crawled off to a forgotten corner of the Galaxy, where his reputation hopefully has not caught up to him, and crawled inside a bottle and takes odd jobs as strictly hired muscle for the local crime lord.
Motivation
Blood Money: Violence is Sting's Business, which is never in short supply. He knows no other way to make money and treats his bloody work as any other job.