Why exactly is it wrong to murder?

I doubt many people think that killing people is always wrong or always unjustified unconditionally. If an insane person attacked someone and the person would kill him in self-defense, I doubt many people would blame him. If the allied troops had captured Hitler and executed him, I suspect that most people would consider that to be justice served. When people say that killing people is wrong, they're usually implicitly referring to innocent people, not serial killers or genocidal dictators. One might consider that a double standard, but it would largely depend on what views on ethics one has, which is a much more complex discussion.