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Sure, not following human morals made life easier for Saya and Fuminori, but isn't this the case for a huge amount of scenarios? The notion that acting in an evil way can benefit oneself isn't a very interesting message for a story to send, hence my dissapointment. Saya's lack of morals doesn't make for an interesting story, because SnU doesn't really comment on it in a meaningful way. Why did Saya choose to embrace a human concept like love, but chooses to disregard a concept like morality? Can humans fall in love with cockroaches and communicate with them in a profound way? It's also likely that Saya is aware of human ethics, but actively chooses to act in defiance of them. Comparing how Saya sees humanity to how humans see cockroaches isn't really fair either imo. Surely it would be evil for humanity to wipe out other species without regret, especially if the species is sentient and intelligent. I can see where you're coming from, but Saya's actions still are immoral based on any form of human ethics. So yeah, it's really easy to hate a lot of media if you look at a problematic story from one side and one side only. Omi, Yoh, Yousuke all went insane when they saw a fraction of what Fuminori saw and while yes, Fuminori did horrible things, only looking at that part of him as a character is unfair.
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Even though Saya indirectly pushed him into doing more and more reprehensible things as the time went by, the existence of Saya kept Fuminori relatively sane. He could never find peace, had no place where he could get a break from all the horrifying shit he saw, essentially his life lost all meaning until he met Saya.
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Not only did he lose his everyday life, but it turned into something horrifying and disgusting, the friends that he treasured turned into slimy creatures he couldn't look at, no matter how hard either party tried. Fuminori lost everything he had all of a sudden. Saya no Uta is one of those stories that make it impossible to like unless one successfully tries to put themselves in the villain's shoes. If you're unable to connect with the characters, of course you're gonna have a bad time.
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