Well, on the show, Kutner didn't leave a note or any other indication of why he did it. I haven't seen anyone write a story that changed that.

I wrote a very short piece about his suicide, Down (http://blackmare-9.livejournal.com/292555.html#cutid1) -- just to sort it out in my own head, and I don't know anyone else who has tackled it. You might try hmd_finders and see if anyone there can rec you something. I'd be interested in seeing someone else's take on it, too.