Claiming moral high ground when you are in the business of killing people seems a dubious exercise in ethics at best. You can quote me on that. But there is a serious bit of ethics that you need to work out for yourself, if you want to be able to live with yourself at the end of the day. Using lethal force is always an ethical decision, and having a proper ethical framework to deal with the aftermath is something you need to put into place.
My bottom line, if you justify your killing as being "for the greater good" you need to shoot yourself next. Fighting for the "greater good" is the justification of tyrants both great and petty.
When you need to justify your actions remember what you are fighting for, which is your own individual liberty and freedom. You don't need to justify yourself beyond that. Killing those who would oppress you, or aid in your oppression, is a simple act of self defense. And there is no justification needed for protecting yourself. The people who lie to themselves and ascribe grand notions to the justification of their actions usually end up needing chemical support (alcohol, drugs) to keep their brain from shouting the truth back at them and destroying their self worth.
In the serious study of ethics there is the cliche "trolley" analogy where you have to choose between acting to kill the fat man and letting multiple others die. The great thing about the trolley scenario is that there are no "wrong" decisions, just decisions that make you defend the ethics of your choice. But here is a better question, is it your place to choose who lives or dies? If you answer yes, then you place yourself in the realm of God and if you say no you then you by definition lose the right to choose to kill someone else to benefit others. But saying "no" still leaves room to kill to benefit yourself, to protect yourself, to protect your intangible interests.
So understand the ethics of war, your reasons to fight should be personal. The communists and tyrants of the world asked in a reasonable manner for everyone to give up "just a little bit, for the common good of course" which lead not to common good, but to common tragedy. When your justifications echo those of the biggest mass murderers in history you need to stop lying to yourself and figure out why you do what you do. "Cause it's fun" is the justification of a monster, but at least it is honest.
There are those who will never have this conversation with themselves. Some people cannot bring themselves to kill, some others cannot help but feel deep regret at the act. On the other end of the spectrum you have those who feel only recoil, or who enjoy enforcing their will on others. It is the people who exist in the middle between those two extremes that need to get straight first.
I can't help you with your personal ethics, I can't tell you that what you choose is right or wrong. At the end of the day you need to be able to look at your reasons for doing what you do and live with yourself. If you need someone else's approval, you need to move beyond that.