Murder and theft in themselves have not been universally bad in the view of all cultures at all times. These often were (or are) evils only if inflicted on outsiders to the group, there being no conception of universal ethics.
Sometimes its not easy for we modern western-influenced people to understand the moral atmosphere even in the “west”, prior to Christianity. In many ways the old pagan world is, to us, inscrutably alien.
Its just taken for granted now. Christianity made a profound difference, even to the irreligious.
If you want to make a try at grasping the nature of pre-Christian morality, have a good look at Homer. His heroes are in fact murderers and thieves, and so are his gods. Homer does not disparage anyone for immorality, but for foolishness and weakness.
Islam overlaid Mohammeds teachings over a pagan culture, that was just as unethical (or subjectively ethical) as any pagans anywhere. In ancient Arabia, raiding warfare, blood feuds and banditry were honorable, as honorable as ancient Rome’s annual organized harassment of its neighbors every summer.
There was much less of an ethical shift than there was when Christianity took over from classical civilization.
And Mohamed, by his own actions, did nothing to change that paradigm of barbarism. He set the example that can’t be replaced or surpassed.
Or you could look at the Code of Hammurabi, Buddhism, Taoism, Hinduism, Confucianism and a host of other codes, religions and philosophies that go back as far or pre-date Homer that quite clearly lay out a strict moral and ethical code of conduct. Or you could simply look to the Jewish canon which pre-dates Christ by a year or two at least.