I would assume probably a similar reason why: 
- God killed the children of the earth with the flood at Noah's time 
- The children of Israel when entering the promised land killed the people's there 
- David killing Goliath 
- Moses himself, descending from Sinai’s summit with the Ten Commandments as fresh in his mind as they were in his hands, immediately caused 3,000 Israelite idolators to be slain 
 
In Nephi's case it states in the Book of Mormon: 1 Nephi 4:13 Behold the 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. 
 
Our earth life in God's eyes are short and temporary. God wants us to return and live with him. Those who are righteous and obedient to God's commandment will have that blessing. Those who don't won't. It was God's love for his children yet to be born that He caused the flood to come to clean the earth of wickedness and provide a better environment for His children to be born in to the world. 
 
Hope that helps.

It is interesting to note that Laban himself pronounced his own punishment when Nephi and his brothers first asked for the plates. 
 
13 And behold, it came to pass that Laban was angry, and thrust him out from his presence; and he would not that he should have the records. Wherefore, he said unto him: Behold thou art a robber, and I will slay thee. 
 
Shortly after that, Laban robs Nephi and his brothers of all of their gold and silver when they offer it in trade for the plates. According to Laban's own words, the punishment for robbery is death. I think it is only fitting that God then commands Nephi to carry out that sentence in the next chapter.

The killing of Laban can seem to be contradictory to the Gospel at first glance, especially if you are unfamiliar with the Hebrew roots of most of the Bible.  
 
Several times throughout all the scriptures killing is denounced. It is even in the 10 commandments "Thou Shall Not Kill". It is important though to note that when the word kill is used in the scriptures it is derived from a Hebrew word that means to kill the innocent, or murder.  
 
So murder is a grave sin, to not all killings are murder. Laban is a perfect example for two reasons.  
 
Art pointed out the first, that Laban decreed (as well as the Lord in Old Testament) that robbers are to be put to death. Laban was a robber and he was put to death by the prophet of the Lord. In a modern society to seems a little extreme to put a thief to death but that was the judicial standard of the time though. So the Law decreed the death of Laban, you cannot murder someone condemned to death.  
 
Also even if you disregard the crimes of Laban that warranted his death under the Mosaic Law, there is still the principals of self defense.  
 
In Alma 43:46 the Lord says "Inasmuch as ye are not guilty of the first offense, neither the second, ye shall not suffer yourselves to be slain by the hands of your enemies". It is important to note that the Lord says two offenses, how many times did Nephi and his brethren go to Laban in peace simply seeking to obtain the Brass Plates? They went twice and twice they were innocent of any wrong, twice there were attacked and would of died if Laban had his way.  
 
In the eyes of the Lord Nephi had no choice but to defend himself against the wicked attempts to kill his family by Laban. The were innocent of the first and second offense and the Spirit commanded Nephi to take the sword in defense of his family.  
 
The loss of human life is always tragic and no one should desire to kill, just like Nephi did not desire to kill (the Spirit commanded him 3 times before he obeyed). Though in the end when there is no alternative one is not committing sin to defend there family and nation from harm as long as they are not the instigating the violence against them.