It depends on what human rights in war we're talking about. There's one set for soldiers and other regular combatants and aoother, much harsher set, for "irregular combatants" and terrorists such as the Death Eaters. I find the Elven Justice against the captured Death Eaters to be very much in the legal tradition of the later set, though definitely on the more severe side of that tradition. Besides, I reckon it's giving the Death Eaters the same regard for human rights, or the rights of any sentient, really, that they've given others; you reap what you sow and they've sown the wind.