Usually is the correct term. Your citation of conquerors glutting on the wholesale slaughter of captives is in reference to a few tribes. Usually, among most tribes, they were treated roughly but humanely and later released, adopted or married into the tribes.
I still do not agree. First, there is little to no reliable documentation about many Native American tribes, so those would have to be excluded and nothing they did could accurately be referred to as "usually" done, "usually" the case, etc. Second, the piece spoke of tribes from regions, the plains tribes, etc. In each region there were different numbers of tribes. What "usually" occurred in one region, or part of one region, might have been dwarfed in numbers of occurrences in another region simply because that region was so much larger and food sources and water sources were far more plentiful and because of that there were more tribes.
There is to much assumption and generalization in your position for you to be able to accurately say what "usually" occurred. The facts are that no one knows for sure so no one can accurately say what "usually" occurred. At most they can say what "usually" occurred within tribes where there is reliable documentation to rely on.
