Yes, slaves do not prosper. As in, their wellbeing is not being addressed. And, as I said, if morality is about anything, it's about the wellbeing of humans (and sentient creatures).
Humans kill each other too. That doesn't make it moral (though it may be moral in some cases, for example, when a person is suffering. There's that nuance again
). But we recognize that it's not a behaviour that optimizes the wellbeing of human beings or of society. In fact, it's a detriment to society if people have to walk around all day worrying that they could be murdered at any moment. It's also a detriment to society if everyone gets murdered, because then there is no more society and no wellbeing for
anyone.
Why do I protest when humans eat other humans?
Well, I do, and I don't. Depends on the situation. If a person is being murdered and eaten, I definitely would protest that. Their wellbeing is completely being taken away from them.
But what if it's a situation similar to one where those Chilean soccer players whose plane went down in the Andes Mountains ended up eating the dead in order to stay alive and attempt to save the remaining survivors? In that case, I would say it's not immoral. The wellbeing of the already-dead person is not being compromised while the wellbeing of those still alive and starving people needs to be addressed.
If Biblical morality is about following orders and commands, then I would have to argue that that is not an exercise in morality at all. That cannot be superior to secular morality, because secular morality involves thinking through our actions and their consequences and weighing out the impact of our actions on those around us. Following commands doesn't involve any of that.
I don't agree that capital punishment is moral. I think it's hypocritical. Once a person is dead, they can't sit and think about the consequences of their actions as a form of punishment. They won't be thinking of much of anything after that. It's the easy way out for them.
This doesn't really answer the question though. I asked what morality is about. Not where or who you think it comes from. It sounds to me like you are saying morality is about following orders from a deity.