Yes but the line we draw between murder and killing is often through man and not God. That's when the problem arises. Unless you are a prophet. It's kind of hard to know whether or not someone is doing something by God's will or not.
I agree with that, man can never be truly certain that a killing is just, we can only get as close as possible, which is the function of a court of law.
That doesn't however mean that you throw the baby out with the bathwater and never engage in killing altogether - this is the heresy of a pacifist society. Pacifist societies never survive because they're trivially conquered. To err is to be human