Let me give you an example. Suppose you kill someone. Good on you: you're a strong human being, having overpowered someone. Suppose you kill someone else, this time more capable of defending themselves. Cool: you're very strong! But then, someone kills you. You lose everything you have ever considered to be yours; you cease to exist entirely: no blackness, no white light - your mind dies with your body. Is it a pleasurable experience? is it desirable? My hope is that you reply with "no" to either and we'll move on. So, consider what those people you've killed feel about the issue: do you think they'll approve you killing them? Some will: some find it simpler to cease to exist rather than face the overwhelming problems they experience; most, however, won't: not only do you lose bad things - pain, suffering, hunger, thirst, inability - but all the good things - joy, laughter, endorsement, achievement, ability, friendship, love, the life you've crafted for yourself, accolades of any sort...

lets take your example and expand and out though and talk about how moral relativism comes into play. Most humans can agree that death is probably a bad thing, but most could also agree that lack of self determination is also a bad thing but what happens when you face the two against each-other? Say you are 16th century colonist and you land in the new world and you perceive that there are a bunch of "savages" killing each-other and doing ritual sacrifices. If your value score for not killing is significantly higher than the value score for self determination slavery may come out as the moral choice.

As we (westerners) look back at history with modern values we may value self determination higher relative to other values and might say that what was done was immoral. We might also expand the choice and say well you know it wasn't really a binary decisions and there were more options that were not really considered. It is quite possible though that the people of the past that we now consider immoral were actually guided by the morality of the time. The same way their morality may have steered them down the wrong path in the past, our current morality may steer us in the wrong direction as well.