I'd argue that we don't,though we'd have to be careful how we define the terms. Some things, like the Holocaust, we can claim are absolutely wrong in terms of contemporary morality, but the Old Testament records tales of mass murder and genocide and what we'd now call ethnic cleansing, some of them done directly by the deity himself, and all of them done with at least his approval and connivance. There's no hint in Scripture that any of them were in any sense a bad thing. Modern ethics would also say that slavery is absolutely wrong, but neither the Bible nor the Quran suggest any such thing, they offer nothing but acceptance of the institution of slavery. Not even Jesus himself, who had plenty of opportunities to speak out against it, is reported as disapproving of it.
The only such reference to I know of in which God himself condoned such warfare is in regards to a group of people who were sacrificing their children to fire, to please their false gods. Such things evidently really ticked God off. He didn't want such practices to spread and contaminate mankind.
As for slavery, that's a good question. On one hand, and I could be wrong here, slavery in those times may have been justified for such a primitive world economy. Plus, I'm pretty sure the good book states that in those times, one must let their slave be free after seven years. Further, doesn't the mass exodus from Egypt, with all its divine intervention show God's desire for such practices to end? He was before the times, he was!;-)
Morality is relative to the societies that define it, and things can be defined as unreservedly bad in those terms, but in truly absolute terms, it doesn't work. I've been thinking about this question since this thread, and some related ones, started, and I've been asking myself, is there something that every culture in history that we know about has defined as absolutely wrong. It may be just a function of my lack of historical knowledge--I'm an engineer, not a historian--but I couldn't come up with anything. Not even murder. Even our own supposedly ethically advanced culture recognizes self-defense as a legitimate justification for it. Is there something in the category of absolute wrong in all societies in all times and places? I haven't been able to think of anything.
Nothing! Not even murder? Notice how the taking of life
must be justified? If morality
really was relative, we wouldn't need the excuse/explanation of self defense for taking one's life. Notice how even cannibals need to justify murder? Why else do they preform elaborate expiatory rituals as a way of cleansing and justifying. They wouldn't do so if unless they thought there was something wrong with taking lives.
Everyone knows murder is wrong. Even mass murders like Hitler had to dehumanize the Jews in order for him to justify killing them. That's not relative morality, that's making excuses for violating objective morality.
If any one of our loved ones were murdered, our REACTION would show the moral code written on our heart. You would condemn it, as an absolutely wrong act. Bin Laden, who murdered thousands of people, knows murder is wrong. Imagine if America flew planes into Saudi buildings and killed innocent people. His REACTION would be to condemn them as absolutely wrong acts.
Even though its violated constantly, we all know murder is absolutely wrong due to the unchanging moral standard.