Naive? I have seen and done more with my life than you could imagine, kid. I'm sorry your worldview does not seem to admit that, just perhaps, the very concept of killing someone who may or may not deserve to be killed, just on the orders of a superior officer, might possibly be an immoral act? Think about it just for a second, before you let your ideas of the nobility of killing someone go to your head. A military sniper, no matter how "good" he might be in normal life, has made the conscious decision that he will, at a moment's notice, kill another person with no more opinion of that person than a "target." That is part of his job and that is what he is expected to do. If he refuses, trust me, he would no longer be a sniper. He might be tasked with putting a bullet into the head of a father, a husband, someone's friend. But because of the need for military expediency, he's a dead man, because his continued life might be problematic. How the heck do you reconcile that sort of performance with someone you want to call good?
Yes, at times, a sniper or assassin might perform a "good" assassination. But don't fool yourself, a sniper in service to any government is not going to be given the luxury of only taking assignments that are morally upstanding. And any person who takes that responsibility upon himself is, by his very acquiescence to the demands of the job, surrendering any moral high ground he might like to think he enjoys. Don't let romanticized, action-movie cliches cloud your thinking. A person who accepts it upon himself to kill someone in a manner such as that is not, and can not be, "good" in the objective moral standard. Neutral, at best.