Halo: Combat Evolved is one of my favorite games, hence the 117.
The robot named 343 Guilty Spark is such a great character.
In honor of 343 days with a cowboy hat here's one of the best "Oh Shit" moments towards the end of the game where you find out the purpose of the halos, spoilers!
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!
The Master Chief : [turns to the Monitor] Is it true?
343 Guilty Spark : More or less. Technically, this installation's pulse has a maximum effective radius of 25,000 light years. But once the 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?
343 Guilty Spark : [to the Chief] We have followed outbreak containment procedure to the letter. You were with me each step of the way as we managed this crisis. Last time you asked me, if it were my choice, would I do it? Having had considerable time to ponder your query, my answer has not changed. There is no choice. We must activate the ring..
Cortana : We need to go. Right now!
343 Guilty Spark : If you are unwilling to help, I will simply find another. Still, I must have the Index. Give your construct to me, or I will be forced to take her from you.
The Master Chief : [raises his weapon] That's not going to happen.
343 Guilty Spark : So be it. Save his head, dispose of the rest.