The Golden Trio and being ruthless.

So, I'm listening to the audiobooks (Stephen Fry) and have been wondering about the actions of the trio when collecting the Hallows/hunting Horcruxes. Basically, when they fight off Dolohov and that other one (the big blonde one), they have them at their mercy and let them go, having adjusted their memories.

Why wouldn't they just finish them off? This is a war and these two are two of the worst enemy combatants. Ideally they would take them prisoner, but that's not an option. Voldemort isn't evil because he kills, I think in these circumstances they would have justification of slitting their throats. Also sends a message to Voldemort.