Considering the fact that they would have benefited more from a British victory, why would any slaves, current or former, choose to fight against the British?
Good question - you have to dig past the superficial to understand, and I'm not fond of the way it is phrased, but the fundamental question is interesting.
Here is one example:
Having fought as a teenager in the French and Indian War, Robbins served in an Acton militia company at the end of the siege of Boston. Caesar Robbins became free during the war. He raised his family in Concord, dying in 1822.
I don't know why Robbins chose to fight, I don't know why his owner chose to trust him with a weapon. (I'm making the broad assumption that "fought..." means that he was armed, although it is possible that he was merely part of the logistics train.) I suspect that the author of the blog above would love to explore that issue. In that sense I can't answer your question, but I can offer at least one example of why someone might want to.
Also note that the concept of slavery changes significantly around Bacon's Rebellion and that the effects of Bacon's rebellion propagate more slowly in the Northeast where slavery is subject to different pressures than in the central states or the South. (discussion of the evolution of slavery).
Having offered my caveats, I speculate that some enslaved people (particularly in the Northeast) may have had more hope of fair treatment from their owners than from the institutions that their owners claimed were enslaving them. Some may have perceived themselves as "Americans" (although enslaved, some of them had been here for generations and had no knowledge of other continents/countries/states). Some may have had hope of emancipation & citizenship - remember the Marblehead brigade included free black men, so people in the Northeast would have had a model.
Some may have been promised freedom in exchange.