Originally Posted by
Kraco
I view it as a temporary boost after consolidating the spirit energy from the 10k human souls. Those souls are needed to become a true demon lord, but perhaps a whole lot of the energy is actually surplus. That surplus would only stay around for a limited time before dissipating. Kind of like in an RPG you could drink a fortify mana potion, which would give +75% max MP, but the effect would only last 60 seconds. It could be used for something requiring an astronomical amount of magic energy. Since it's a demon lord evolution, that is, restructing a monster to become a demon lord, perhaps that energy is especially attuned to handling souls and corporeal bodies, meaning it's also useful for resurrection.
Sure, I also agree on killing soldiers who surrendered as being evil, but it doesn't matter since it's something he needed to do. A ruler must be able to do evil things as well, when necessary. I said it before, but war is fundamentally evil business. It's all about killing and destroying, no matter for what reason you are doing it. Yet it's something that absolutely needs to be done when you are defending yourself. If you are conquering foreign land, it's the price to pay for that fell act. What Rimuru did was also defending his country by bringing back dead casualties. It was a bad act for a good purpose.
War is a situation where the only realistic choice is to fight fire with fire. A situation exactly comparable to this wouldn't exist in RL, so it's kind of hollow to really analyse it with our ethics. Perhaps the closest one would be a sinking ship with both your own troops and enemy POWs. You arrive at the scene with a small vessel with only enough space for either your own people or the enemy prisoners. Would you save your own, some of both, or the enemies?