For the returners, I think their way of life and thinking is different from normal humans for the most part. As in when they were human, they despaired and gave up on their human life by slicing their throat, to maybe become a returner or die, so they can stop being themselves as a human but instead their past self which is a great.
And I don't remember some stuff, but wasn't like a goal for new returners was to like completely become their past selves as in complete returners and forgot about their human self. And if they do become complete returners, I mean from what we see now they are sort of immortal unless killed somehow, which for some of them looks to be almost impossible, and for the most part they also can't fit in with normal society I think, so they can either go into hiding forever or go all out with their power, which for some doing so would end the world.
Which is bad, but due to how the returners originated and depending on whether they are a sinner or a great, I don't think they care too much about good or evil, killing people or not killing. And the only precious thing about them is their talent which they traded their life for, if you take away their talent they would just be plain old humans which they dislike enough to kill themselves over.
Kouu arc is on sinners also wanting to be good, but each of the returners has their own thoughts, feelings and desire, which for Alexander I feel like based on the little I know of history, they think what they do is right(good) I mean like how he became King of Conquerors and "undid" the unsolvable Gordian Knot. So if her actions result in the end of the world, for her I feel she would just think "It is what it is".