: I'm inclined to think that the vacuum is lethal to Flood, in that we've never
: seen them operate in a vacuum and they have to have some method of
: respiration. But my guess is given what we learned in the Forerunner
: Trilogy, combined with little details throughout the original games, that
: the spores themselves might be a different matter--able to remain inert or
: dormant in the absence of stimuli or a place to reproduce.

: 343GS mentions evacuating all matter from contained sections in the CEA
: terminals, but it's not specified in what capacity this action is
: done--perhaps it's not to kill Flood, but to quarantine them to a certain
: section by depriving it of areas to expand to.

: As to why 343GS didn't do these things in CE (aside from the fact that none
: of these capabilities had been established)... there's several options.
: One is that he actually did attempt it. After all we only visited a tiny
: portion of the ring, and it seems likely the Flood could have spread
: pretty rapidly. I'd say that the control room diagram of the ring suggests
: as much (I can't think of what else those red zones indicate.) But perhaps
: things spiraled out of control so quickly, and the presence of not one but
: two vessels that could allow them to escape the ring forced him to
: escalate protocol.

: The other possibility is he wasn't thinking straight. Or it could be both in
: equal measure. We know Spark wasn't all there long before CE rolled
: around.

I agree that things spiralled out of control quickly on installation 04. Weren't the humans on the ring scattered over a large area as well, with the escape pods landing all over the place? If there were little pockets of humans all over the ring, the flood may have found multiple vectors once it had been let out by the Covenant forces. Evacuating may have had little effect once the spread began.

Just playing through the library level in Halo CE it's obvious early on that Guilty Spark is a little off. He talks like he knows you and that you should know him as well, and he keeps humming to himself as he flies around. He's eccentric at the very least, and more likely borderline insane.

A combination of quick escalation and a mentally unstable manager is a plausible explanation to me.