"How MUST an Omniscient God Behave?"

This rhetorical question was asked recently by a believer suggesting that as finite persons we cannot fault God for allowing the evil we see in this world. Here's my response:

You realize that whatever you say in answer to this question is telling us how an omniscient God must behave, right?

But I say that an omniscient God should be able to understand us as human beings such that he would be able to communicate with us better than he did in the Bible. There is no doubt in my mind that an omniscient God could've communicated in such a way that the Church as an institution would not have santioned killing people who didn't believe like they did in the Inquisition and witch hunts (which killed innocent people). And there is no justification for such a God not to have unequivacably condemed slavery either. If your response is that he has higher, mysterious purposes, that we simply cannot understand, then such a God should know that since we cannot understand him we would also reject him. And yet this is what I find you defending...that an omniscient God has acted contrary to his expressed desires that we should accept him.