Horatio death is completely unbelievable. So enbelievable in fact that I don't think its real. Is it possible that he staged his death? Hear me out.
He could have sensed that there is no way he could get to his enemies when alive, so he had to "die". The "its done" message could have come from anyone but certainly not from the guy in prison. My guess is that its from H. Also it would explain why Horatio's supposed dead body got cleared. For him to make people believe he is dead, the people closest to him must believe it.
That is one theorie but certainly open to challenge.
Go on and kill some time before the premiere.