Denton and his crew's target was Marcone. The person who gave Denton the belt might have been targeting someone else or had some even more mysterious goal in mind.
What is interesting is Marcone thought he was working with Denton, until the last minute. Yes, I get that Denton was after Marcone, however that doesn't answer some really important questions..
1] Who or what was Denton working with that would draw the attention of someone like possibly
Cowl? The description in Harry's soul gaze of Denton strongly suggests that it is Cowl.
2] The soul gaze had Denton kneeling in front of the robed figure to receive the belts. That suggests perhaps that Denton was a devotee or follower of Cowl, therefore Kemmler, not unlike Kumori. When Harry confronts Denton about the killings including the sabotaging of MacFinn's circl to cover their crimes. Denton answers they were necessary "sacrifices." That is a fanatic with a cause, again perhaps not unlike Kumori.
3] When Denton and his men started killing, "the lobo murders" as they were called, someone warned him that if he kept that up, they'd draw the attention of the White Council. Who was that?
4] They targeted MacFinn to cover up their own crimes. Though Denton wanted to get Marcone, the belts made that easier, but I seriously doubt that was the reason he was presented with the belts in the first place.
5] Giving the belts to mere vanilla mortals was a serious miscalculation of whoever gave them to Denton and his men. My theory is whoever gave them to Denton thought they could then control them and use them as "enforcers." The miscalculation was the effects of the belts on the wearer, the infusion of black magic, power, and blood lust immediately made the wearer go out of control and uncontrollable. Even for Harry, strong willed wizard, who had some idea of what the belts did, was hard pressed to recover from their effects.
6] Conclusion, it was an experimental direction that resulted in a dead end.