What is death? The clinical definition might reference the heart muscle or brain functionality, but what about it makes death so central to human law. It is criminal in most countries to commit murder in some fashion, but why?
Atheists really cannot explain why. They think human kind is the byproduct of random chemical reactions. Our existence is neither special (self-serving identification is no identification at all; assume a character in the video game “The Sims” identified themselves as having value), nor is it of any objective worth (as opposed to subjective worth). Why then, does the atheist oppose murder, besides a theistic hangover?
Regardless, Christians have their own view of death. The Bible describes death as a transition between this life and the next, not to be feared by Christians:
1Co 15:54 So when this corruptible shall have put on incorruption, and this mortal shall have put on immortality, then shall be brought to pass the saying that is written, Death is swallowed up in victory.
1Co 15:55 O death, where is thy sting? O grave, where is thy victory?
But then why is it wrong to murder someone?
We are made in the image of God (Gen 1:27), giving us inherent value. Destroying the image of God, without justification, is wrong. We generally do not have the authority to transition people between states of existence. God alone reserves that right, which he delegates to the government (Rom 13:4) in certain situations (executions, war) and even to individuals in certain situations (self defense, blood-revenge killings). Otherwise, we have no right to transition other people from the various states of existence.
One Christian publication described it as if someone owned a bike. It is ok for him to disassemble it, but not other people. It is all about authority.
As a Christian, I do not fear death, but I recognize that mass murder (Stalin: 60 million, Hitler: 20 million, Mao: 80 million) is evil because human beings have inherent value and those murderers had no authority to ruin those people’s lives, transitioning them into eternity.