1 Nephi 4:18
Therefore I did obey the voice of the Spirit, and took Laban by the hair of the head, and I smote off his head with his own sword.
Wow, this verse out of context to one who does not know the story might be a bit startling! Laban was a very wicked man. Nephi and his brothers wanted the scriptures to take with their family, so that they could have the word of God. Laban didn't want them for their spiritual value. And he robbed the brothers of their belongings and threatened to kill them. Nephi prayed for help on the 3rd attempt, and upon entering the city, he found a drunk Laban and received a command from God to kill him. Previous verses (12-17) give a little reasoning behind that:
We have seen in our history how one man can cause so much evil and harm (e.g., Hitler, Osama, Hussein, Kaiser Wilhelm, particular leaders of my own country whom I shall not name right now). I'm not saying that this would have been an easy commandment for me to follow. Probably one of the hardest for me. You should have seen me in our debate in my junior history class, on the side against the death penalty. Human life is precious, no matter what that human has chosen to do with theirs. I don't feel it is our part to play God and decide who lives or doesn't. And even if He were to command me to take a life to save millions in a nation, it would still be one of the most difficult things of my life. I hope I am never faced with a command like that.
But I am faced with other commandments that are hard for me. Perhaps not as equally hard. But they are tough enough for me to follow. Yet I know it will be worth it for me to obey them, and that I will be greatly blessed in the end if at least for being obedient.