Thou shalt not kill.
As mentioned by others, "kill" is "murder".
I am perhaps straying a bit around, and outside the scope of the question, but you might find some of the essence useful. Beware of some doom and gloom ;)
In short, it is God's prerogative to end a life, not man. Though I do not say God murder, the commandment is set forth to man, not God. If Abraham had set forth to kill his son on his own accord, that would have been a sin, as he was commanded by God to do it, disobeying would have been a sin. Though; as we know this was only a test on Abraham's obedience and faith, not an fulfilled act.
Taking a step back one have to remember that we live in a world of death. By fall of man sin entered the world, and death trough sin, thus the world and all living things die. Even the creation itself withers away. The creation that was perfect became corrupted. The world is full of life and joy, but surely full of death and suffering as well. This is part of the fallen world. God does not enjoy it, thus he set forth to save us.
God is almighty and we can not even come close to fully understand God, his nature and his actions. We can, however, see that God throughout the Bible, is a loving, merciful, wise, just God. One can also surely say he is a patient and forgiving God. That is the "norm" so to speak. So why then does he "kill people"?
As we see from context man sometimes fall under God's judgement. Perhaps especially when the society they live in are completely rotten; to the core (Gen 18).
And the Lord said, Because the cry of Sodom and Gomorrah is great, and because their sin is very grievous;
The cities cries to the heavens. One can only imagine their evil and bad ways. Still, we remember from the story of Sodom and Gomorrah that Abraham pleads (Gen 18):
And Abraham drew near, and said, Wilt thou also destroy the righteous with the wicked?
And he said, Oh let not the Lord be angry, and I will speak yet but this once: Peradventure ten shall be found there. And he said, I will not destroy it for ten's sake.
– If he had found ten righteous, the city would have been saved.
Another "big" example is the tribes of Canaan. (Gen 15)
But in the fourth generation they shall come hither again: for the iniquity of the Amorites is not yet full.
– iniquity is here perversity, moral evil, etc. This shows that God knew they would become even more perverted and evil, but they were not there yet. The people of Abraham was not to take the land until the iniquity of the Amorites was full.
We also know that Israel was punished when they strayed away from God.
Beside this it is part of the fallen nature. Death entered the world. The world is no longer as it should be.
One also have to remember how serious the act of killing is in God's eyes. Even David, who fought a righteous war, was marked (1 Chron 28):
But God said unto me, Thou shalt not build an house for my name, because thou hast been a man of war, and hast shed blood.
We learn that life is precious, and the life is in the blood. Blood, even from animals, has it's place. Thus is one of the old ways emphasized in (Acts 15) by telling us not to consume blood.
But should write to them to abstain from the things polluted by idols, and from sexual immorality, and from what has been strangled, and from blood.
Looking at Genesis 9, we read that
Whoso sheddeth man's blood, by man shall his blood be shed: for in the image of God made he man.
By the laws given the taking of life was a capital offense. Even killing by accident was punishable by death, though one then had the possibility to save oneself by taking refuge in dedicated areas. One was permitted to kill a thief, but only by night when one could not tell what the intentions of the intruder was. Sheree murder in any form was capital offense.
Throughout the Bible there are several passages about defending one self, rescuing others etc. But one are never to kill by e.g. revenge, anger, be quick to draw sword etc.
Finally, in short, one clearly see that God say Thou shalt not murder. If all types of killing, even self-defense, just war or even capital punishment was a sin, then one would end in a circular fallacy. It is clear that it is murder or even negligent homicide.
Some notes further on.
One often hear that "it was the God of the Old Testament", but the seriousness in it is a reality today as well. I truly believe the world is different, as with the spiritual world, after Jesus victory over death, but, God still see to nations and people, as well as the individual. So even by remembering (John 3:17)
For God did not send his Son into the world to condemn the world, but in order that the world might be saved through him.
And by knowing though we live in an imperfect world, we have salvation and God's love and peace in our heart, we live to serve and rejoice even trough suffering (Rom. 5)
Therefore, since we have been justified by faith, we have peace with God through our Lord Jesus Christ. Through him we have also obtained access by faith into this grace in which we stand, and we rejoice in hope of the glory of God. More than that, we rejoice in our sufferings, knowing that suffering produces endurance, and endurance produces character, and character produces hope, and hope does not put us to shame, because God's love has been poured into our hearts through the Holy Spirit who has been given to us.
God's wrath as well as his blessing is as real today as it ever has been.
My point in mentioning this in context of the question is the reality of the fallen world and the importance of prayer for our nation, leaders, people, as well as family and communion.
I believe God can hold his hand over and bless nations, as well as take that blessing and protection away. That is serious business.