system
Given the following post, respond as if you're commenting on Reddit:
human
[Halo] Why can't the Halos target the Flood directly?

If the Forerunners from the Halo series could create the Halo array to destroy all sentient life in the galaxy to starve the flood, why couldn't they use the halos to target the flood directly and not any other life forms?
gpt
Killing pretty much all life in a Galaxy in and of itself was a major undertaking.  Tuning the process to effect just the flood may not have been possible while under direct invasion from the Flood.   Further more the flood infect their "food" to make more flood.   Even if they could kill the parasites, they may not have been able to kill the infected  "food."

The flood are an Extra-Galactic threat.   The Forerunner decisions to wipe out all life, including the bulk of their civilization, was one not just one last FU to the gravemind, but as of saving other Galaxies from the fate of our own.