The fact that ethical norms vary between cultures is not evidence of the absence of universal moral principles. I think that such universal principles do exist, and can be discovered beneath the superficially variant customs/morals practised by different groups of people.

These principles are of necessity very general, and their implementation/interpretation will vary widely across different cultures and time periods. Take this principle: 'thou shalt not cause unnecessary harm'. Whether we're talking about the Witoto tribe or the United Kingdom, this principle is in practise. In both cultures, if one goes on a random killing spree, they will be excommunicated, killed, or incarcerated.

Of course, the definition of 'unnecessary killing' will be very different between the two cultures. So will the kinds of entities who are deemed to fall within the sphere of ethical consideration. Many other qualifications of the principle will also vary. But that doesn't change the fact that the same intuition is operative in both cultures: the intuition (or idea, whatever) that causing unnecessary harm is wrong.

If someone wants to refute what I've said, I'll be happy to hear it. My mind isn't 100% made up on the issue, so I'd like to be convincingly refuted.