Didn't Mishima suffer from manic-depression (bipolar illness)? This illness alone was probably responsible for his death by suicide. Incidentally, manic-depression (bipolar I especially) was often mistaken for schizophrenia in those days.
If you study the biographies of great writers, artists, poets, and musicians who took their own lives, you'll be surprised at how many of them suffered from bipolar illness, I or II. Lord Byron, Sylvia Plath, Anne Sexton, Ernest Hemingway, Virginia Wolfe, van Gogh, etc. Here is a good book on the subject: "Touched with Fire," by Kay Redfield Jamison.