People are not stupid, most of them. So justifying Kana's actions over and over does not invalidate the matter at hand.
They are not saying they don't understand her behavior. They are displeased by it. Understanding someone and condemning their actions are different. Kana's actions are not rocket science.
Regardless of her reasons, what she did, for various reasons, can be inherently offending. Her actions vs her intentions. Her actions is the turmoil for some. And clearly, many seem to have deep rooted issues with corpse slappers.
If Aqua was my own son and that wannabe slapped my child at his funeral, hatred would not be the only feeling I would spare her. Regardless how much sentimentalism one wishes to paint this action, what she did was very tactless.
Finally, I am not suggesting that hating Kana should be the default reaction, but questions like "why is she being hated" are pretty... void, because the answer is blatantly obvious.
Personally, Kana's bratty attitude has always pissed me off since day one. At this point, I am used to her rudeness knowing no limits.
People are hating on Kana for that? Bruh obviously people don't get that was to show how much his death affected her and how close they were. Is like making a promise to your closest friend, lover or whatever, maybe as a joke, but he goes and actually dies, she is obviously sad and mad, if she didn't show emotion at all that would be worst.
Everytime I went to a funeral the people closest to them always made a "show" is uncomfortable for the other people there, but absolutely real.