People are just mad they didn't get a cookie-cutter happy ending from a show that OBVIOUSLY would not have a happy ending. The first three characters they introduce all instantly die. No way anyone was expecting anything other than a main character dying
People are mad, because 160+ chapter, supposedly complex story and with amazing character build-up ended "flat" as a crêpe. Story ended without any moral whatsoever, one would argue some of it's points even contradicted themself, i.e. all that suicide take or introduction of crow girl and whole plot around her.
But what's even worse, is how author smashed his carefully and painstakingly made characters to the ground with only those few last episodes.
Aqua, who's whole build-up was the most prominent, ended as he begone - with only slight change, that he regretted for a mere second what he has done. 160 chapters of trying to accept who he is, leaving his old-self and revenge that this piece of Gorou inside him carried, only to end up sacrificing himself for the sake of "saving his sister", while it was only to satisfy his narcissism and self-loathing, which were brought from his previous life? Sounds like Aqua from ch. 40, not like Aqua from ch. 140.
Same with Ruby - character build around suffering and agony, experiencing even more dreadful things through her second life, slowly falling into even bigger misery for over 16 years, saved only because fate smiled on her once in her life. Then, after losing everything again, she just get up after it, left alone on this world, like it was nothing.
I could yap and argue for hours how bad this ending was and why it was so shit, but I won't, cause it no longer matter.
But no, people aren't mad, because it wasn't all cutesy and fluffy ending. It's not about Aqua's death per se either. If you really think, that people are giving this story negative feedback with such shallow reason in mind, I guess you are the one, who doesn't understand what was this series about and what we could expect and get at the end.