So murder is fine

What's fine mean? You're just using it as a standin for "good" or "acceptable" which again, has no meaning without a subject. Fine for what? Good or acceptable for who?

most of the world doesn’t agree murder is morally wrong

What?

Your slavery example doesn’t work because slavery was and is always immoral

Then why, for hundreds of years, was it considered so moral in America that it took an entire war to change it? Because most people individually all just happened to lack this obvious, natural morality? Or is it because ideas of morality are made up, arise from material conditions, and are systemically reproduced in a society? That they were able to create a system of moral justifications that excused them in their own minds for supporting slavery, just like liberals today have a system of moral justifications that excuse them in their own minds for supporting the Gaza genocide?

You seem to think that "right and wrong" exist in some pure elemental form somewhere out in the universe, the way carbon and potassium do, some baseline that human actions can be measured against. They don't. Right and wrong are figments of the human imagination, and only by accepting them for the shadows that they are can we stop chasing them in circles and finally begin to think in useful, concrete, scientific terms.