Why did Nephi murder Laban?

The Spirit made him do it.

I was constrained by the Spirit that I should kill Laban. 1 Nephi 4:10

The Spirit said unto me again: Slay him. 1 Nephi 4:12

It is better that one man should perish than that a nation should dwindle and perish in unbelief. 1 Nephi 4:13