Glenn Greenwald wrote an excellent summary of certain facts concerning the anthrax attack suspect that killed himself. It highlights the fact that most media claims concerning his instability comes from a social worker who isn't terribly credible, especially when everyone else had a very different impression of him. It strengthens my suspicion that Dr. Ivins, the suspect for the anthrax attack, was an innocent researcher on whom the pressure from the FBI investigation drove him to suicide. However, I don't think the investigation was the main impetus for his suicide; it's more probable that it had merely aggravated an already existing mood disorder. Even if the social worker was telling only the truth, it could easily have been due to a depression made worse by the investigation, rather than him being psychopathic from birth.
Of course, I'm rather removed from all the facts and the above is largely speculation, but I think it's most likely. It's still entirely possible that he was indeed the perpetuator of the attacks or that he was murdered in a way to look like suicide so that the case could be officially closed. In any case, all the evidence that has been made available to the public so far is purely circumstantial; the FBI would have to reveal really strong evidence to convince everyone that the case should be closed.