Theistic morality

[Purple Rabbit]

The fact it's an objective fact that slavery in general causes an awful lot of suffering to the slaves in question, demonstrates the immorality of it I would say.

No, it doesn't. That fact shows nothing but the fact itself.

But your statement shows that you attribute values to it that go beyond the fact itself:
v1) it is wrong to cause suffering to slaves
v2) all wrong-doing to others must be labeled "immoral"

It also suggests a rationale accompanying it, somehing like:
r1) slaves are human individuals that can experience feelings and emotion
r2) resticting freedom of human individuals will be experienced as maltreatment
r3) every human individual will benefit from a general rule to avoid maltreatment of anyone

And it suggests a goal to strive for:
g1) Avoid all human suffering

So the label "immoral" more or less reveals your personal values, your personal rationale behind it and your personal moral goal you've set.

So far so good. We can inform us by science about the rationale. We can investigate with science whether slaves in any way are intrinsically distinct from free people. We can inform us with science in what ways a general rule for avoidance of maltreatment will bebefit individuals in a group. And so on.

Now suppose someone (let's call him Mr X) with the following values and rationale:

Values:
V1) It is not wrong to cause suffering to slaves
V2) Slaves can be owned as property

Rationale:
R1) A person brought into slavery transforms into a human subclass
R2) Slavery can enlighten suffering of free individuals
R3) Maltreat of slaves is necessary to subdue them

Moral goal:
G1) Avoid suffering of me and my family as free individuals

Mr X will label it immoral to not maltreat slaves.

Please observe that my description is not very far off from historical fact. Science can help to disentangle R1-3 but it cannot produce an objective truth for g1 over G1 or vice versa. Science can inform us about the efficiency and effectiveness of obtaining a certain goal by certain means but the goal has to be presented up front and will always involve a choice made by man.

---

It's a tentative shared informed standard and that's all it takes

So, if we had a tentative shared informed standard that slavery was alright, would it be morally acceptable?

Indeed it would be and historically it has been in large parts of the world. See the example above.