Like every career and job, you're going to meet a few who are idiots. Everyone who has joined the military know, or should know, they are forfeiting a lot of their freedom and will to serve, so to say that they are "mindless automatons with no will of their own" is a given when it comes to taking orders and completing it. Beyond that however such statement becomes invalid as soldiers do have a mind and will when it comes to how to make theirs and their unit's life and mission easier.
As for whether or not soldiers do any self-reflection on their role, their task, their actions. They do, but they can't do it too much or seriously as it is detrimental of what they must do. How does a human cope if they have to kill other humans and they must kill? Some don't think about it. Some demonize their enemies as being lesser than humans. Some rationalize it such as "kill or be killed". So long as the soldier is not hesitant to pull that trigger when he or she must, whatever works.
Indeed they fight because they are ordered to fight. That's how the military works. The military is an arm of the government, it should not have a mind of its own beyond having a mind of how to punch faster, smarter, and take blows from other arms. The military is not, and should not, be an institution that can direct its own action and policy, and that includes the soldiers within the military.
As for the reasons why people would join, some of the reasons that I know:
1. Stable pay or for education benefits
2. To be in the military. They like the military (when they joined).
3. Explore the world, being part of something big, being part of an institution that is changing the world. Basically making their life bigger and fulfilling.
4. They couldn't think of anything else. This is good if the person didn't have much opportunities to begin at no fault of their own. This is bad if the person didn't have any choice by their own faults.
5. To serve the host nation. I'm serious. Some join because they somehow feel obligated to serve.
6. Family-oriented reasons: family tradition, family or relative killed or wounded in war or attack.
7. Career outside of military. Some join to get experience for another career in the future. Some might join intelligence or military police to go into law enforcement. Some might want to join aviation to become a civilian pilot. Some might join to gain leadership skills for business administration.
8. Character Building. Some join to empower themselves, whether to prove themselves that they're not physically weak, courage-deprived, etc.
I have yet to meet ANYONE who has joined because they want to kill or shoot weapons.