You miss my point. I understand what the obvious motivation is. But actually killing a man is emotionally quite hard, especially when he's shooting back at you (this is why even some good hunters fail in these situations, there's a classic scene in The Last Samurai about this). One of my favorite gunslinger quotes is "You don't have to be fast, you just have to be willing."

Even in those times you mention, not everyone was up to the task, and not just because they weren't sharp shooters; they just weren't willing in the above sense. Think about it: Would you be able to ruthlessly kill a "bad guy", when if he shot at you first.