In the pages of Final Crisis 1, Libra, the mysteriously resurrected villain kills the Martian Manhunter with a spear of flame. The death is so short that it only takes up one panel.
Now, knowing Grant Morrison, I know that lots of important things happen very quickly in his densely plotted stories. Try reading his run on JLA, and tell me that it isn't ridiculously densely plotted. So I'm okay with J'onn being killed in one panel. That's not what throws me off. What kind of makes me ill is that we know that J'onn survives until the 853rd century, in which he has become a sentient desert on Mars, while he waits for the Resurrection Man and Vandal Savage to fight so that he can give them the "kryptonite" which is actually a Green Lantern ring. Not only does this rob the Final Crisis storyline of some resonance, but it also robs DC One Million of its power. And, uh, also, J'onn is awesome. Easily one of the best characters in the JLA. He's mysterious, inhuman while being the most human, likes Oreos, and is powerful enough to be a big gun. This is me being an annoying fanboy, I'm afraid. It could all be for nothing - we know nothing of Morrison's longterm plans for Final Crisis. Still... I'm going to go re-read DC One Million to drown my sorrows.