>>538098753
It has more to do with the trope of a character vowing to protect another. Very classic shonen manga dynamic between the hero and the heroine, it's basically a love confession in itself. So when a character like that dies, it hurts. You've probably seen how in serial mangas and anime they usually hesitate to kill of characters because it can make a reader base disappear if it upsets them enough. In the final PV3 of VV before release they blatantly spoiled that Aogami lives and even put that right on the thumbnail of the video for the Japanese version so you would see it without having to even watch it. This was the PV following where they showed the new form, heavily implying something bad happens to Aogami. They were probably afraid if they made it seem Aogami stays dead people wouldn't buy VV. Honestly with how some people were reacting that might have happened.