Nephi decides to sneak back into the city to get the brass plates, because asking Laban nicely didn't work and paying him fared even worse. Nephi discovers Laban passed out drunk.
Nephi Kills a Defenseless Sleeping Scumbag
Laban was a bad guy. But, as I've already explained, he didn't have to die.
Assuming You're Called of God Makes an Ass Out of U and Zoram
When Nephi, wearing Laban's clothing, manages to get Laban's servant Zoram and the brass plates outside of the city, his deception eventually falls through. Zoram realizes that Nephi is not, in fact, his boss, and he tries to bolt. Nephi grabs him, and convinces him to stay with his family with this gem of an argument (verse 34): "Surely the Lord hath commanded us to do this thing; and shall we not be diligent in keeping the commandments of the Lord?"
Perhaps this sets the precedent for Mormonism's attitude of assuming its own veracity. Nephi's comment to Zoram is basically saying, "God told me to do all this. You want to do what God says, right?" He focuses more on following God than on whether or not God actually told him to do it. It's behavior that feels very much in line with Mormonism's emphasis on obedience and not with verifying the source of the commandment.
I guess Nephi was just a few thousand years ahead of his time.