Why is it allowed for nephi to slay leban and not repent?

Answer 1

If you read the passages where this story is related, you would see that God commanded Nephi to slay Laban, because "Lord slayeth the wicked to bring forth his righteous purposes. It is better that one man should perish than that a nation should dwindle and perish in unbelief." (1 Nephi 4:13)

If you are familiar with the Old Testament, you will understand that several times, the Lord commanded the righteous to completely destroy the wicked. For example, in Numbers 25, God commands Moses to smite the Midianites. In 1 Samuel 15, God commands Saul to destroy the Amalekites, and to "utterly destroy all that they have, and spare them not; but slay both man and woman, infant and suckling, ox and sheep, camel and ass." The Book of Deuteronomy also has several stories of the Israelites destroying entire races by the commandment of the Lord.

This is the world Nephi came from. These Old Testament Israelites were his ancestors. Nephi was literally living in the middle of the Old Testament, his father Lehi was a contemporary of the Prophet Jeremiah. The plates he was sent to collect from Laban contained these stories, and he understood why he was to do what God commanded him to do.