It's roughly the military equivalent of the enemy having their artillery emplacements overrun and then turned against their own troops. Size of the respective fighting forces is irrelevant, it's simply a useful tactic. Of course if the overrun emplacements were part of a large, belligerent force agitating a smaller force without cause, there may seem to be a certain justice in using their weapons against them. It doesn't make the subsequent killing "good", but it may help discourage further aggression against strategically disadvantaged but tactically superior forces.

Sometimes.

The greatest good is our aim here.