I disagree a little, everything you described would only prove why the author's writing is good, in my opinion. Like, I wouldn't say it's the author trying to convince us that the childhood friend was selfish, but rather the characters who have the point of view on death that thinks she's selfish, and since they think this way, they will inevitably talk about her this way. And well, I didn't like the way she took out all her frustration on ML either, just because she couldn't turn Shiina into someone normal. I know she cares about Shiina but she knows very well what kind of person Shiina is, so much so that she had this goal of "turning her into a normal person", so there was no reason for her to put all the blame on him. Shiina was definitely not normal before he arrived, she was just pretending.
And the reason Shiina is the way she is is because of her mother, as shown in the manga. Her mother literally killed herself in front of her when she was a child and told her that life has no meaning and that she should pursue death... And if you know the slightest bit about how a child's brain works, it's obvious that they'd be this fucked up when they grow up. Especially after meeting people like her "first love" who only fed this fucked up thought of hers.
I found the characters well written and the plot interesting. I just didn't understand the ending tbh
i don’t care what anyone says, i really disliked this manga. everyone is so shallow and boring. especially shiina. her whole thing is literally just “boohoo look at me everyone! im obsessed with the idea of death” and it’s genuinely for no reason. like ok hoe we get it. i thought that maybe there’d be an understandable explanation later on in the manga as to why she idolizes death so much but there isn’t. what’s even worse is the fact that she just dragged the ML into her bullshit and traumatized him further. the only characters who actually made sense to me was kaede and shiina’s childhood friend. despite what kaede was going through she didn’t drag the ML into any of her problems and truly only wanted what was best for him. i hate how the author tried to portray shiina’s childhood friend as a selfish person when she too only wanted what was best for the person she cared for (shiina). she wanted shiina to get better, she wanted her to live. that’s what real love looks like.