This is an interesting conundrum. Effectively, Q would be giving Riker et alles the tool for a Borg genocide. That would be a heavy thing to contemplate, indeed, especially since it was later proven that some Borg can split off and retain some of their pre-assimilation individualism. Would it be genocide? The Borg is not one species since it's made up of many species from around the galaxy, but it could be theoretically defined as a single, unified, culture (albeit totally alien to us). I suppose that an argument could be made that if other members of assimilated species still exist, it wouldn't be a complete genocide. However, if there are one or more species who were assimilated in their entirety without any non-assimilated members left in the universe, then it could be a multi-genocide.
And what if said button-pressing would be another secret test by the Q? Meaning, if Riker (or Worf) presses the button, that guarantees that the Federation is still far too dangerous to be allowed to exist themselves, leading to their own destruction by the Q? Ugh...I think that would actually WAY too dark and heavy for a TNG episode. DS9, maybe, but not TNG. I honestly don't know how that would play out and you would need some seriously talented writers to un-paint them out of that corner.
