The manga really touches on this concept. You should read it. In the anime it really gives a different feel to the story. Saya does acknowledge she is a chiropteran at one point but it fades away because she really wants to be human. In the manga Diva shows her love for Saya because Saya want to fight so she agrees to make her sister happy.

No, Saya is not selfish. Diva poses a threat to mankind therefore she must be eliminated. I don't get what's so wrong to get rid of her. Diva is killed in the end and that's that. You're kinda like saying that the police should try and understand criminals when their job is to arrest or kill (If the criminal poses too much of threat) them.

I kinda feel for Diva in both anime and manga: all she wanted was to love and be loved. It's because of twisted works of humans that she became such in the first place. But I won't call Saya totally selfish: she was spoilt (because of humans again) at first and when she realized about her circumstances she wanted to kill herself too, didn't she?

It's because of nurture that they ended up in such state. Still I won't totally blame human nature 'cause in the end they are the ones who helped solve all issues.

Saya is somewhat selfish in the anime. But in the manga, Kai asks her if she was going to kill Hagi and Riku because they're Chiropterans as well, and she says yes without a second thought!!!! I was like D:<<<<<<

Now that I think about it, at LEAST she was planning to kill herself.... But in the anime she was very overdramatic about it in episode 49.

No, Saya was not selfish. Despite the fact that Joel was a bastard for keeping Diva locked up, she still murdered a bunch of people and Saya was doing what she thought best in that situation. And Saya does acknowledge she is a Chiropteran; it's just that, unlike Diva, she doesn't think being a Chiropteran means having to be a homocidal monster who kills hhumans at the drop of a hat.

I don't think that Saya was selfish--far from it, she fighted and passed through lot of crap (and psychological trauma) just to protect humankind of the Chiropterans. And even if she came to understand Diva, she still had to stop her. Diva, as much as I love her and think that her personality was a result of negligence, torture, as well Amshel's manipulation, was a serious threat and with the Riku incident she proved to be beyond redemption/rehabilitation, perhaps if she was removed from Amshel's influence and given a good therapy (from whom? I don't know) she would had gotten better, but at the end, there was nothing that could had been done about her, except killing her.

However, I have to agree that Saya IS narrow-minded regarding Chiropterans. She has a black and white view in that aspect, but honestly, I can understand why: From what I have read from Adagio manga, Saya from the get-go was concientized that she had to fight to stop Chiropterans and Diva, and even the early member of the early founded Red Shield planned to kill her in case she lost control (even if at the end they didn't), because of that, adding to the fact that she lived enclosed in the Zoo with no other human contact other than Joel and Haji for her first 50 years of life, plus being so focused in her war with Diva that she may had overlooked (through the historical periods she lived on) that humans also can be monsters, thus she had this view that "Chiropterans = evil, humans = good". It was Kai who, both when they encountered the Schiff and when she planned to kill her nieces and herself, opened her eyes and made her see that being a Chiropteran didn't make her evil, and that she and the babies had every right to live like any other living creature. One of the aspects that intrigues me of a potential sequel is how will be Saya's life and views now that the fight is over.

So yeah, she wasn't selfish.

Selfishness never came into the equation. It's simply that Saya was raised as a member of humanity, and bestowed with human values, while Diva was arbitrarily locked away as a monster. This difference led to Saya placing value in human lives while Diva, who was not raised humanely, doesn't regard herself as a member of humanity and so doesn't value them. Taking all this into account, the entire conflict was caused by a philosophical difference caused by Saya being conditioned to develop Pinocchio Syndrome (https://tvtropes.org/pmwiki/pmwiki.php/Main/PinocchioSyndrome), and value her 'fellow' members of humanity above her own life to the point of planning to murder-suicide herself and her sister for their sake.

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.