The Merriam-Webster dictionary defines murder as: "the crime of unlawfully killing a person especially with malice aforethought." By this definition, this would clearly mean that lawfully killing someone, as compared to unlawfully killing someone, is not murder. The problem with that type of flawed thinking is that the State's laws may not meet with moral law. That is to say, if you live in some sort of country, to which certain crimes, are sanctioned for death, as given by the laws of that country, than you as an instrument of that country, are permitted to kill, without having to worry about being later indicted for murder, even though you have taken another human being's life.
In America, members of the armed forces, certain other associated military and governmental personnel, as well as law enforcement officers, are either explicitly permitted to take another person's life under certain circumstances, or implicitly allowed or even encouraged to do so. That sort of power, that is to say, the power to take away life, is a power that trumps all other rights, because without life, you are no longer of this world.
It is always a lot easier to kill people, if first you demonize them, trivialize them, ostracize them, or do just about anything to make the case that these people appear to be something less than a human being and thereby not really deserving of a fundamental right to life. That is to say, if you strip a man of his humanity and dignity, then you are that much closer to transforming that man into prey.
All of these things to which lethal force is used against another human being, to which, that killing force is neither necessary in order to defend oneself or necessary in order to defend one's country are in a significant amount of cases, morally wrong, whether they are legally sanctioned by the State, or not. The big picture that most countries and many people seem to miss is that murder has been part and parcel of how human beings, tribes, and countries have treated each other through the eons of time, and all of that killing, and all of that bloodshed, and all of that misery, has yet to bring peace and harmony to families, communities, and countries--and it never will.
There is a mistaken notion demonstrated by the actions of basically good men with reasonable common sense, that evil, or the bad guys, have to be stopped with bullets, no matter the cost, and therein lies the problem. There is a cost for every bullet shot, there is a cost for every action of revenge, there is a cost for hatred for your neighbor, and that is as long as you participate in the cycle of killing, by approving of it, aiding and abetting it, or participating in it, than the killings and the murders will never end.
Murder is like an argument, to which, one party just has to get the last word in, and they wrongly believe that by killing a fellow man, the dispute has ended there, and all will be good. But, you can't end murder by murder, you can't end killing by killing, you can't end any negative action by responding in like, because it will end nothing. The only possible way to abolish murder is to walk away from all of the killing, to not participate or partake in any aspect of it, but instead to make your life an exemplar that each of us is granted life by our Creator and that each of us is birth with "…the kingdom of God is within you" (Luke 17:21) to which that kingdom is a world without beginning and a world without end, of which murder is the trade of those who are left behind.