You don't even get a single standard with God either, that's why there are more than 40,000 sects of Christianity, all of whom think God wants different things. Luckily, for those of us who live in reality, we realized that standards are rarely ever eternal, things do change as societies change. You might not like that but that's the way it is.
You condemn God but indict men. God's morals are the exact same, your talking about moral epistemology (how we come to know morality, not moral ontology (the actual nature of morality). I don't care if none of the 40,000 sects got every moral hey believe in wrong that has no impact on what God's morality is. What a thing is does not depend on how I come to know it or even if no one knows what it is. Pluto was an objective solar body before anyone had ever heard of it.
God's nature determines what morality is and his nature does not change. His commands are merely reflections of his nature. I could have predicted your ontology epistemology confusion and I will predict the next misstep you would have taken and respond before hand. God's moral nature never change but his commands could. Moral commands have two parties but his nature only one. We change and so the exact same moral nature may produce two rules. The exact same way we teach kids to do one thing and then another when they get older but retain the same nature ourselves (in this sense anyway). For example don't eat pork was a rule given when no reliable way to cook it enough to kill its parasites existed but that rule went away when we could but God never changed.
As for denominations having different ideas. 90% of denominations believe 90% of the same things. The differences are usually about procedure not morality anyway. Having a piano in church or not is not really a moral dilemma but a procedural quirk. However it does not matter what or how many groups believe in differing things the nature of the thing (morality in this case) would be unaffected. Murder would be just as wrong even if no human on earth believed it was given God. The objectivity of a concept does not in any way depend on the awareness of what the objective fact is.
Is slavery actually wrong or just socially unpopular at the moment? It depends on when you ask Christians, they have justified slavery or hated slavery at various times in their history.
That one is a little complex semantically .
Chattel slavery is objectively wrong but voluntary servitude was permissible. God wanted neither but like divorce he did allow things because of our sin. Their was no welfare in the bronze age and slavery was universal. God created the most benevolent slavery laws on earth at the time. I am getting a head of myself. The word slavery does not appear in the bible as written. You first need to look up the words that were translated as slavery and see what they meant. Fortunately I have debated this issue many times.
1. The bible words translated as slavery never mean chattel slavery.
2. The word slavery in modern times is loaded with 20th century baggage that does not apply to the ANE.
3. The bible's wording I similar to servitude but can range from voluntary to permanent in extremely rare cases but never chattel slavery.
4. The only records that survive are all of voluntary debt slavery. A man would find himself in dept. He would voluntarily agree to serve a man who would pay off his entire debt. The man had property rights, could marry and have a family, was given a home and food and had legal protections. Even if his debt was not worked off he went free after six years anyway. This was biblical servitude as practiced in general but there were more harsh types.
5. One was war prisoners. At that time the normal action was to kill them all or let them roam around preying on their neighbors for food etc.... Refuge laws and the UN did not exist in the ANE.
6. Another was one that is not mentioned much but is when a person who already owned a slave would sell them. Being foreigners freedom would have caused the same problems as the above. No doubt these occurred but were so scare no record of them survives.
Let me give a few more comments and be done here.
1. Slaves could legally escape at any time.
2. It was illegal to turn them in (contrast that with Babylon which was the exact opposite).
3. They could live in any tribes land in Israel which not even the Hebrews could do.
Summary:
Slavery was allowed by God because our sins and our capabilities at that time made it the best solution for a terrible problem. It was not what we think of as slavery. In modern times the most Christian nation on earth is the only example of a significant nation self condemning Slavery. It was defended by a few rich Christian politicians and senior officers in the south but the South's soldiers were not fighting for slavery but for the state's right to make up their own minds (which is what we fought the revolution for), and 300,00 Christians died to set men they never met free. Find another example of that in history.
If you think a single immoral act is actually wrong it takes God to make it so.