Plenty of people joke about off color things. But, from my experience, even in private, they generally don't joke about things they could do. So I might joke about drone striking or nuking someone. But who cares, I have no power to do either of those things.
However I don't joke, even in private, about introducing backdoors into my code. The reason why is because it is something I actually can do.
So yes, people take part in dark humor all the time in private. However it's usually about unrealistic things. I don't know exactly how to resolve this with particularly powerful people though, so I would err on the side of giving them a pass.
First, the Secretary of State doesn't control the drones. It's not her call to make.
Second, a drone strike in London is not realistic. It may be technically feasible, but it's not politically feasible. It would wreck diplomatic relations with basically the entire world.
If an email leaked from my CEO with such a joke in a "private email" - I'd be looking for another job. Even if it was "obviously a joke". Just because it was said in private doesn't make it any less bad if the public hears it or change the standard people will judge it upon.
I would fully expect a CEO to joke about layoffs and whatever else during meetings with top executives. Doing it in front of the whole company would be horrible.