Simple Post: Michael Altman was a planned martyr.
Follow me on this; what better way is there to have your ideas embraced by multitudes? Martyrdom. If Altman died natural causes, a lot of the fervor dies away - no savior or rally point. If Altman commits suicide, well, who wants to join another suicide cult? (Though one could argue that once one has joined, one isn't leaving.) But...if Altman is "assassinated", that implies that he knew something special. That the big, bad, government silenced him to suppress his movement.
The best way to inspire your people to unheard of levels of fanaticism? Die gloriously, cut down, slain defending his beliefs....you've got a movement, quite an impulse to such a movement.
The best thing Altman could have done for unitology was....die such a death.