They're especially useful for defending against innocent civilians decades after the war ends.

Clearly you haven't been paying Fallout.

Depends on how you use the gun. But that's the thing about landmines, you "plant" them rather than "use" them, and pure fate alone is the arbiter of the mine's "use." Landmines kill indiscriminately, they don't care what team you're on, how old you are, if you're even a combatant at all or just a 6yo civilian child, but a gun at least requires a person to make the decision to act or not. It is for this reason that I contend while guns can be (not always are) used morally, landmines cannot, even when they do happen to find a "correct" target.