I do not believe it was stealing, as many times I've openly offered bounties of that order of magnitude. He merely pulled it from me, rather than wait on me to push it to him. And that's fine by me, what he found was clearly worth a lot more than that.
In my experience it is hard to get paid properly for an exploit, so he helped himself to what he considered his work to be worth. That's questionable morally, of course, but he put a lot of work into developing the exploit code and wanted to make sure he was suitably rewarded for it. He reported it to MoneyPot in a responsible manner, and shared his exploit code once it was fixed.
I'm not sure what motivates him, but if he ever wants to claim credit and add it to his list of achievements, I wanted to be clear that I bear no ill will, nor do I feel wronged (but rather the contrary).