I was speaking metaphorically. It's the same way Mab gets Harry to do what she wants or the way Lash got Harry to kill himself.
My apologies, due the nature of the debate previously I assumed you were being literal. And I quite agree, Mab's ability to manipulate and influence and force people to do what she wants is second to none in the series.
Even if that were possible - pretty sure Demonreach wouldn't be a fan, the question is why Merlin and the following Wardens chose to imprison rather than kill all those nasties. We know Naagloshii are immortal, but I suspect the one that Morgan destroyed was merely put down for a time or else would have had to be blown up in a special place in space or time. My guess is either Merlin and his following Wardens didn't know how to kill immortals OR they thought trapping them was far easier, more efficient and better overall.
Even assuming you could live with the guilt of murdering millions of people and causing untold destruction yourself by blowing up Demonreach on Halloween - that energy of all those immortals has to go somewhere, it can't be destroyed. Mother Summer confirms this in Summer Knight. So I would guess it either goes out to Oblivion or Outside (assuming they are different places) or that energy goes into the Nevernever and something or a group of somethings get a whole lot of power to add to their own. You quite likely create a problem worse than the solution, is my point.
Not to mention, not only would Demonreach not cooperate with this plan (not in the least because it too would die, but it's primary function is to stop this event all together) but I imagine Vadderung, most of the Gray Council and several other interested parties wouldn't allow it, you know like the guys who primarily don't want to see an apocalypse. I would say there are a fair few (both good and bad) who are in the know and wouldn't like that outcome.
Kumori, Cowl and several others have had a distinctly different flavor of dark magic. My theory is that Harry's own bias has colored how he sees Dark Magic, and isn't aware of the mechanisms and subtleties all that much. Meaning if he knew the source of black magic, if he knew how corruption works, he might understand why Kumori and Cowl and himself have a different feel of black magic too them. And consider this: Justin was a bad guy using black magic, but he had been hiding his power while he was still considered a Warden. Not only that but Harry or Elaine would have felt the wrongness of Justin's magic if he had been horribly tainted, yet they didn't. Meaning there are nuances and tricks that Harry is blind to. Even Peabody quite likely hid his magical taint (assuming he had some). We know this is possible from WOJ.
Arjan, I think it's a lot more grey than that. I don't think it is so black and white as if you use too much you are a Warlock. The Council has definitions that don't necessarily match up with the universe's. Jim said something about that when people asked him about the Laws of Magic.