The characters literally disagree with you and I'll go with what they say since they know their entire life story and we don't.
Theyd on't dissagree because I'm not making and excuse for anyone's wrong doing. But the fundamental difference between Mimi and Nana against Sierra feels understated. We clearly see how far individualism goes until someone imposes their will onto you and you're working one step away from slavery. The coercion of it all is pretty significant. Working in proximity of what could happen to them if they were to act out of line is pretty significant.
That is a chasm the size of the grand canyon away from being indoctrinated by the nation around you, that you're implying is Sierra's circumstance. Sierra who committed horrible acts to Nin'you just like her knowing that she'd never be put in that position because of her own privilege. If your customs is systematically killing off a group of people, the only reason you think its normal is because its not happening to you. That's an explanation but its not an excuse like being forced into actually doing harm or you'll die, starve, or be put out with nowhere to sleep or sold off like cattle. Sierra never had to worry about that.
Yes both of them had choices they could have made to not hurt others but the conequences to those choice don't even come close to being the same. The circumstances of those choices don't come close to being the same. And the people who delt with them have different levels of compassion. Do I think Kagura would have spared Sierra... who knows, she was there to help with the death of Mimi and while Kagura goes out of her way to try and not kill anyone, she'll do it. Do I think Cattelya is wrong to kill Sierra? No, she was complicit in the death of not just many Nin'you but Cattelya's own subbordinates. You asseted that Cattleya would kill Nana but that doesn't make any sesne when no one died in that part of the mission. Be distrustful, sure, but Cattelya still hasn't been show to be a thoughtless killer for no reason. Knowing what we know, Sierra woudl killed Mimi and Nana faster than Cattelya would for her own greater good.
I never got the impression that Sierra is just a kid. She seems just as old and Raito and Kagura because she knows the differne between right and wrong ands he still made a wrong choice from a place of privilge to chose somethng elese. Even if that something else was just not participating instead of activly going against her father. Her making that choice is hundreds of times easier than the choices that Nana or Mimi would have had to make. I'm not saying that Sierra doesn't have to compacity to change like Nana and Mimi did but her conequences aren't outlandish for the choices that she made.