If Bible sanctions capital punishment, it of necessity sanctions those who enforce the laws to take a person's life. That occupation is open even to Christians. Therefore, a Christian can be a soldier or police officer. Murder pollutes a nation. Bloodshed pollutes the land, and atonement cannot be made for the land on which blood has been shed, except by the blood of the one who shed it.
Murder affects the productivity of a nation.
Your brother's blood cries out to me from the ground. Now you are under a curse and driven from the ground, which opened its mouth to receive your brother's blood from your hand. When you work the ground, it will no longer yield its crops for you. You will be a restless wanderer on the earth. Then the LORD put a mark on Cain so that no one who found him would kill him.