Right now I'm reading Ishmael Beah's book. He was a child soldier in Sierra Leone in the early nineties. The reason child soldiers are so effective is not because of their 'skill level' - they aren't better trained than other soldiers, they aren't stronger or faster, they're just completely willing to kill you. And most people aren't willing to kill a child, even if that child is running at you with a machete.
I think a lot of modern military training tries to train this, but there's only so much you can do in a theoretical framework. In the real world (and maybe dwarves are different, I don't know) there are a whole lot of psychological blocks in place that stop us from hurting each other, let alone killing each other. It's why people get traumatized in war situations after they have to kill someone, even in self defense. I have some martial arts training, but I'm not a very good fighter because (while I have a pretty high pain tolerance) I can't force myself to hurt someone else.
Basically, you can teach the raw skills, but those aren't going to matter one bit if you involuntarily pull your punches. And I think that it's actually easier psychologically to kill someone today, with a gun, than it is/was with handheld weapons. You don't have to force a chunk of metal through the muscle and gristle of another screaming, flailing, emphatically alive human, you just have to pull the trigger.