Tricky one.
Ignore that you're talking about nukes, and let's just say that you have a choice of killing 5M to stop 8M deaths.
A lot will come down to who the 5M people are. In past wars (e.g. WW2) "soft" targets were generally acceptable - that is, it was ok to bomb cities with wholly civilian populations. It's something that was done may times, though usually because there was, somewhere in the middle of it, a political or military key target. If you did that now, though, you would be in serious trouble. The weapons exist to not have to carpet bomb cities to hit a single building.
But, if you bombed 5M enemy combatants (I'm not sure why 5M troops would all be in the same area?) then that would present a target that might well be seen as a legitimate.

So I think the upshot is that if you killed 5M non-combatants today then you'd be the bad guy. If you did it 75 years ago then it's more likely to have been accepted (not by the people who got bombed though!).