Hunter's suicide is an extremely classic case of the suicide of a manic depressive. Just about the right time of year too though a tad early. Most manic depressives who off themselves tend to do so in either March or April. Also, writers are extremely highly represented in the ranks of the manically depressed.
When a manic depressive does kill themselves, it's just at the turning point of just coming off of a manic cycle. You will often hear the friends and family make claims of surprise because of how happy they were at that time. The belief is that they can't handle the roller coaster ride back down the hill or that dreadful crash into despair.
Whether Hunter was a manic depressive, I honestly don't know but I would say that it's a fairly safe bet saying that he definitely was.