Only reason Heller might not be dead... No silent countdown.
Oh wow, I forgot about the silent countdowns!
TV Guide interviewed Manny Coto and Evan Katz (producers of 24) about this episode. You can read the whole interview here, but this is the relevant section:
You killed the president! Unless you want us to believe he might have survived that missile exploding...
Manny Coto: No, it's pretty cut and dry.
Why did Heller decide to give himself over to Margot despite Jack's insistence not to?
Coto: When Heller invokes the fact that he is ill, that he has Alzheimer's and he is fading away, that makes Jack stop. Heller's reasoning is actually not a bad one in the sense that, "I only have a certain amount of time left. And if I live, more people will die. ... If I could prevent that, what am I really giving up? I'm giving up a couple years — maybe one more year of lucidity." [In Heller's mind] it's worth the trade.
Evan Katz: It is a very strong statement of heroism and nobility and sacrifice. How many of our presidents would do that? We loaded the dice in our 24 way, where he knows he doesn't have that much time left, and he thinks this woman will keep her word.