Murder is defined as unlawful killing, its a specific word that is used and was created and made up for that very purpose, go look it up in a dictionary
I really dont know how many times you want me to give you my opinion on where morality comes from, just because you either cant grasp or wont accept my reasons is lost on me at the moment, maybe you are trolling or just dont see whats in front of you..
And where do inalienable human rights come from and who ensures you have them?
You can keep repeating the same thing over and over, but it's still wrong. Manslaughter is considered an unlawful killing. You fail to understand that murder is a specific subset of killing. That is the premeditated taking of a life of an innocent person. That's another example of your shallow thinking. And it still does not answer the basic question as to how do you know it's wrong.
But this post does allude as to why do you think murder is wrong. It's not because it violates some moral imperative. In your world, there is no objective morality, only opinions. So it's just your opinion. The concept of right and wrong can just as easily be called "like" and "dislike". And these are subject to whims and feelings of the time.
Saying murder is wrong or evil is just the same as saying "I dislike murder". And then we are left with moral relativism. There is no absolute morality but it's only relative to the individual. So you can have rules and laws, but that doesn't stop me from deciding to murder if I think I can get away with it and if in my opinion, I don't think it is wrong. But knowing I will have to answer to this sin and believing there is more to this life and this world then just what you see around you, it is a huge incentive to keep me on the straight and narrow. There is an especially egregious homeless person in our neighborhood that I wish was dead. If I didn't believe it was a sin I would put a bullet in his wretched head and go to sleep knowing that I, in a small way made, the world a better place. And no one would give a shit but breathe a sigh of relief. I doubt the murder investigation would be priority one for L.E.
So with moral relativism, we have what we are having so much of today: moral confusion. So, if you want a good world, then the loss of objective, absolute, universal morality will only make it worse. "Without God, all is permitted."