Morality and legality, killing and murder
With regard to any human action, the principal question for Christians is not whether it is legal but whether it is moral; which is to say, at least according to one tradition of Christian ethical grammar, whether it is in accordance with God's will.
If an action is not in accordance with God's will, if it is immoral, then it is wrong to do—full stop.
If it is permitted by God's will, or licit, but not commanded, then it is right to do, but not wrong not to do it.
If it is required by God's will, or commanded, then it is wrong not to do it—full stop.
In instances of the last case, whether or not the action in question is permitted by human or civil law is irrelevant. Indeed, whether or not the letter of the law calls an action "X," it remains "X" if the moral law—that is, the will and command of God—deems it to be such.
For example, if a man is killed in cold blood by another man—if, that is, the killer is solely and personally responsible for the other man's death, being the natural result of the killer's freely willed and intentional agency, and the killer lacks any reasonable justification for it—the only Christian word for that action is "murder." Whether or not the legal authorities have the same definition for such an action, and whether or not the relevant legal procedures find the killer in question guilty of murder, the action remains, morally and theologically speaking, murder.
"Murder," in other words, is the divine judgment on the action in question, whatever human judgment the civil law declares it to be. And the latter bears not one iota on the former, which, for those who fear God, is the only judgment that matters.
Christians would do well, when reasoning in public, to bear these distinctions in mind.