What I know immediately is that it hurts me.. I don't know immediately that it hurts you; that is a deduction I make based on the similarities between us. And there is no deriving "ought" from "is." The fact that my act hurts you doesn't imply that I ought not do it (that I am ethically bound not to do it).
As to the lack of immediacy of my knowing that another suffers, I don't see the point: Yes, I do infer
that there is suffering and I know what it is because I too have suffered and I know the tell tale signs: the grimace, the groaning and the rest. My knowing in this case is not at issue. It is reasonable, merely.
But the ought from an is, now this is to the point. I am a moral realist, or an ethical objectivist. There are other names for this, I am sure. I explain above what I mean by this, in a fast and loose sort of way, but pretty accurate I think: Pain is what I will call a quasi fact, or a "queer" fact (a borrowed term from John Mackie). Now, Wittgenstein stated that in a book of all possible facts, some hypothetical all encompassing compendium, the fact of pain would be included, but the badness of pain, the moral dimension of pain, would not, for such a thing cannot be witnessed (this is how I take it). I disagree. I think there are moral facts; further, I think such things are absolutes and they are the foundation of the presence of morality in our existence. To me the matter is simple, if unpopular: pain (value) possesses a non natural quality which is moral "badness". The awkwardness of the term shows how unfamiliar it is, given that such a thing is generally just freighted along with god and religion, and these stigmatize the genuine absoluteness of what is plain and simple. All one need do is observe the pain of the spear in one's kidney and understand that there is something there, something sui generis in
the pain. The objection of is vs ought is a red herring that obscures the obvious.
Given that the pain hurts and that hurting is grounded absolutely then one is eo ipso bound to not hurting others.
I recommend dismissing the notion of god, the myth. Such a thing just muddies the water. I know, the OP says god, but I can take this term as I please, and I please to reduce god to its material essence: experience.