From what I have gathered he was a proponent of old ideas of Japanese honor and imperialism and was to some extent disgusted by the intrusion of Western ideas into their culture. I believe part of the motivation of his "coup" was to restore some element which he felt had been lost in the government of Japan after WWII. He had a list of demands, which he read to a crowd which had gathered below the Diet building his team had occupied, and apparently he was jeered at or ignored, or simply not heard at all, and disgusted with this reaction he killed himself.

Interestingly, his second, whose job it was to cut off his head, was apparently shaken by the whole event and unable to complete the blow. Someone else had to step in and finish the job after the first cut hit his shoulder or left his head dangling.

Legolas is right in that he was a bit fixated on death before this event and it was in no way an impulsive gesture. He had previously had photographs taken and films of himself miming the seppukku ritual.

I have a biography of him that I really ought to read. On the other hand there is a fairly interesting book by a British guy called Mishima's Sword, which is about the author's attempt to find said artifact which was used by the Tate no Kai member to cut Mishima's head. It's kind of a memoir or travelogue but has interesting information about Mishima and Japanese culture which one might not find in a typical biography.