Why did the Spirit command Nephi to kill Laban?

 

Why did the Spirit command Nephi 

to kill Laban?

According to the account, one of the first things Nephi noticed after he found Laban passed out in the street was his sword. He pulled it out of its sheath, noticing as he did so that the hilt--the handle--was of pure gold, and that the blade was of precious and likely very durable steel. It was night time, but there must have been some light, because Nephi could see it was Laban, and also the details of his sword.

Then the voice of the Holy Ghost spoke to him with a startling command: kill Laban. Despite the fact that Laban had tried to kill him and his brothers and stolen his family's substantial wealth, he didn't want to kill Laban. Being the good young man that he was, he recoiled. He had never killed anyone. Then the Spirit spoke to him again: "Slay him, for the Lord has delivered him into your hands. The Lord slays the wicked to bring about His righteous purposes. It is better for one man to die than for an entire nation to dwindle and perish in unbelief."

Nephi thought about what the Holy Ghost had just said. His family needed the Brass Plates to have the Law--God's commandments--so they could know and obey the Law. If they didn't have them written down in a lasting record, they would forget them, and their descendents would eventually lose the true knowledge of God and the blessings that come from doing His will. 

After contemplating this, Nephi obeyed. He grabbed Laban by the hair and cut off his head. This must have been very hard for Nephi to do. But he also realized that if the Lord commanded it, it was necessary. Once Laban was dead, Nephi took Laban's clothes and put them on, including his beautiful sword. Still following the impressions of the Spirit, he headed for Laban's treasury to get the Brass Plates.