The Master Chief: The Flood is spreading. If we activate Halo's defenses, we can wipe them out.
Cortana: You have no idea how this ring works, do you? Why the Forerunners built it? Halo doesn't kill Flood, IT KILLS THEIR FOOD! Humans, Covenant, whatever; we're all equally edible. The only way to stop the Flood is to starve them to death, and that's EXACTLY what Halo is designed to do; wipe the galaxy clean of all sentient life. You don't believe me? Ask HIM.
[points to the Monitor]
The Master Chief: [turns to the Monitor with a serious demeanor] Is it true?
343 Guilty Spark: ...More or less. Technically, this installation's pulse has a maximum effective radius of up to 25,000 light years. But, once others follow suit, this galaxy will be quite devoid of life. Or, at least any life with sufficient biomass to sustain the Flood. But you already knew that. I mean... how couldn't you?
Cortana: Left out that little detail, did he?