I don't really understand why people are worried. I don't think the "falling in love kills vampires" thing was honestly something that actually happened. Kiku died in the sun. Maybe love weakened her. But she died in the sun.
But we have also seen vampires sitting in the sun just fine - for a limited period of time, anyway. Remember when they chased down Kiku and it became morning? They weren't burning up. It was also explicitly noted that freshly turned vampires are particularly weak to sunlight - which indicates that the author felt it was necessary to justify why Mahiru would die quickly where other vampires didn't. If Kotoyama feels the need to justify that, it's suggests that Kiku
shouldn't have died from such limited exposure. Of course, he also provided counter-evidence in that she was weakened.
I'll say this: given the number of times Kiku tried to fall in love, my personal suspicion is that this may very well be a myth. Surely she couldn’t have failed to find love with
literal centuries of trying. It just seems phenomenally unlikely, and I wonder if Mahiru’s dialogue at the end, “did you have fun,” was acknowledging that she’d failed for the final time. Asking her if, even though she’d failed to die as a human, did she enjoy the time she spent with him. But I don’t know.
On top of that, Kou is half vampire. Unless I missed some damning evidence, which I don't think I did, there is very little real support behind the love theory. It was just what kiku believed. Nazuna's mom thought differently.
There is some support. It's the fact that the two vampires who we
know fell in love with a human are both dead as a consequence of their love. Kiku died in a lover's suicide, and Haru died in childbirth because she wouldn't drink the blood of the human she loved, and refused to drink the blood of another human. So, like a lot of myths, there could be a grain of truth in it. Being in love makes it much harder to survive as a vampire.
Also, it just occurred to me that there
is some compelling evidence for emotional attachment being what weakens them. The fact that religious icons like crosses worked on formerly christian vampires strongly indicates that it's the
emotional attachment which hurts them. The physical cross isn't tied to their past life, but the emotional attachment to their faith
was a part of their human life. If that's the case, it'd partly explain why Haru was so reticent to suck the blood of her lover. Part of it would be not wanting to curse him with immortality
and having him forget her - though all the vamps seem to remember who turned them, so I'm not sure about forgetting. Anyway, a contributing factor could be that, as the object of her affection, his blood would be poison to her. But drinking from somebody else would be unfaithful. So she chose to starve. Guess we'll get a test of that soon if Nazuna wants a snack.
IDK. I think there's at least room for debate. It's not so cut and dry.
