Kizuki's sucide

Kizukis' decision to kill himself isn't very well explained. We don't see much of him, even though he is responsible for all the drama that follows. In fact from what we now of him there isn't much that would suggest he is depressed.

This ignorance about the cause for his suicide is something shared by the characters and it torments them plenty. However a little hint about what could have happened had Kizuki not killed himself is made. Nagasava is described as some one who isn't meant to or wants to be happy by Wantanabee. Much like Kizuki. In another similarity Hazumi and Nagasava also share what you could call a sexually dysfunctional relationship.
There is no certainty that Naoko wasn't devoid of any psychological problems before Kizukis sucide. Even if Kizuki had, say, gotten himself institutionalized who are we to say that this would be a futile effort just as it was with Naoko?

The scene where Wantanabee dreams [?] of the two women in his life together under a tree. is very symbolic. His only other dream- of Kizuki- was dreamt when he faced his fears/ apprehension and went to meet Naoko. The second one after he finally is able to deal with his past and becomes fully interested in Midori.

His decision to worsen the cut on his hand or to keep it around is very interesting. You don't really hear that much introspection from him at times. Since most people who cut themselves do it in order to to help feel alive, or just feel something, it isn't hard to see his confusion and apprehension about having to deal with his ever increasing list of problems.
That is what the whole film seems to be about a look back on how he managed to deal with his problems [and some of his friends didn't] and what he feels about it.