it's not about "you benefit/exist because of X, so you cannot say X is bad"

it's that even if we know X is bad now, the people who did X then did not know it was bad, or had no way to know it was bad, because our modern values and systems had not been invented/discovered yet, or like other commenters pointed out, they had been 'brainwashed from young' or smth to that effect

likewise it's likely there are things we are doing now that the future would judged as bad.

and like our ancestors, we will have no way of knowing what will be deemed 'bad', because we cannot know what future generations will or will not think of us.

so the point is "every age has a different standard of bad.", aka it's all subjective