Board Thread:Questions and Answers/@comment-1962215-20131014224458/@comment-39010520-20191103025537

Put simply: Saya doesn't place any value on Chiropteran lives. To her, they're all monsters who slaughter innocents.

In both the manga and the anime, it's pretty obvious that her plan is not to kill Diva only - it's to kill Diva and then to kill herself. Her end goal is the extermination of her own species, and she only relents at the end after being confronted about it. It probably helped that the people she would have had to kill were children.

I think her decision is probably fine. Yes, without Saya and those children, there may be no further Queens in the world, but if those wine stores shown earlier in the series are all full of Diva's blood like we expect, then there's no shortage of material for human scientists to create new Chiropterans even without a Queen. Without a queen, there's no way to kill them consistently either (though I suspect a method could be developed). I think it's fairly safe to say that the Chiropteran threat is too large to be contained at this point. You would need to kill all scientists around the world who know about them (and destroy all their samples), and after the outbreaks, there's at least a handful in each major country who know.