Is one of your premises that morality is something that has an objective scale and ours is better developed than theirs?
Non-agrarian societies around the world tend to have cultures/religions more similar to each other than to ours, in part because it's more advantageous to how they live their lives.
As an example: a lot of hunter-gatherer cultures have a way of committing what we'd consider infanticide. Agrarian cultures would tend to frown on that since "all humans have a soul", but the reality of moving around with limited food means that you have to find some way of controlling population.
Another example: the way we treat livestock might be appalling to hunter-gatherers since "animals have souls too". But farmers need to raise and slaughter herd animals to survive.
> uncontacted tribes live outside of national and international laws
Also consider that maybe they have their own laws that you and I are in violation of, who's laws should we follow?