The Guardians

Summary
In both Halo 1 and Halo 2, when a player in multiplayer is killed inexplicably (i.e. by a physics glitch, by the environment, or by an AI in Halo: Custom Edition), the message reads "_player_ was killed by The Guardians". There are also ways to do this among players (guardian glitch). For example, aim at a teammate's head with a Battle Rifle (or a Pistol or Carbine), right trigger and Y simultaneously until he dies. Another way is when a player is killed by a melee attack from another player who died just as they did the melee attack. Also being killed in & out of maps by unmanned vehicles has been known to cause this and has resulted in an unexplainable maniacal laugh being heard on occasion. Another more simple way to see the message is in the level Terminal. If you stand in front of the Train and let it crash into you the sign will pop up at the corner of the screen. Also on the level Zanzibar if you can manage to stand on the windmill blades all the way to the top (an easy way to do this is to constantly walk backwards as the windmill blades ascend), you will be caught between the windmill blade and a pocket at the top of the inside of the windmill and you will be killed. You can do this with a Warthog as well, but no one must be in the Warthog, ways to do this, is to drive a Warthog at top speed and get out before hitting someone.

There is also a third way, doable in Halo 2. Dual wield Plasma Pistols and have any weapon on backup. Charge both plasma pistols, and a split second after you release the charge, press B. If done correctly, the second pistol will disappear instead of drop, and you will still be holding the two pistols on everybody else's screen. Pull out your secondary weapon, crouch(to prevent shooting at yourself, sometimes you do that), and open fire on a player. The bullets will seemingly come out of nowhere, and when you kill the other player, it will say "Killed by the Guardians" on their screen.

Finally, there is a fourth way. Sometimes when doing the superjump on Zanzibar by the windwheel, you will fall through the ground into a ocean underneath the map Zanzibar and once you hit the water, you die and the message says "Killed By The Guardians." This was after the auto-update so Bungie presumably made it that no one can cheat in matchmaking. But its actually rare that this event happens.

This message also appears in Halo 3. When viewing a Service Record and going into Career Stats, your kills with each weapon and viewable along with an image and description of said weapon. One of these weapons is displayed as a "?", with the tooltip displaying "Killed, once again, by the Guardians."

Halo 3
In Halo 3, the "Guardians" are the objects that keep players from leaving the boundaries of several "open" maps, such as the plasma cannons on Snowbound or the land mines that surround Sandtrap. In addition, on the map Narrows, it is possible to fall off the lower bridge onto the structure far underneath. The play will stand there for several seconds before being killed by the "Guardians."

The terminals found in the campaign allude that the Guardians are the Forerunners.