I've never experienced suicidal thoughts, so I can't be certain, but when I read stories like this which are basically aimed at telling suicidal people that they have reasons to live, it feels like it would just come across as patronizing and preachy. I genuinely believe that someone could theoretically be emotionally and mentally stable and still decide to commit suicide (I'm not sure why they would want to, but that would be their choice). However, the situation this manga portrays is not that. Misao wants to kill himself because he's unable to cope with his regrets and trauma from the past, so in that situation trying to help him come to terms with it and cope as Maria does is probably the best course of action. But in her attempts to cheer him up, she minimizes his responsibility. She says "It's not your fault I ended up like this," but...the thing is...it kind of is his fault. Now, obviously, he's not responsible for her actions. But he is responsible for suggesting her actions as well as encouraging and even pressuring her into taking those actions (he knew she wanted to play with them, so he should have known that she would've felt incentive to do what it took to do so). People have to learn how to cope with their guilt, but the way to cope is not by glossing over your mistakes and acting as though you weren't responsible when you were. So in that sense, I feel that this manga has a shallow and badly considered emotional resolution, which could potentially instill unhealthy coping skills in the readers.
Also, it has to be mentioned that Maria is a hypocrite. And her entire pro-living message just goes up in smoke at the end.