Killjoy Medal

From Halopedia, the Halo Wiki

Killjoy is a medal awarded and achieved in Halo 3 Multiplayer. To obtain this medal you must kill somebody who is on an Killing Spree or any other known spree (Killing Frenzy, Running Riot, Rampage, Untouchable or Invincible). It is represented by an octagon with a crossed-out smiley face in the middle. This medal is very random as you don't know which players are on killing sprees, unless of course, one has killed you or your team multiple times without dying. A good way to see this through is to look for a sniper who is sniping other people. Go to an inconspicuous location and snipe him.

The Awww, Too Bad achievement can be gained by getting a Killjoy on one of the Mythic Maps.

	Halo 3 Multiplayer Medals
Special	Perfection • Extermination
Sprees	Killing Spree • Killing Frenzy • Running Riot • Rampage • Untouchable • Invincible
Weapon Sprees	Shotgun Spree • Sword Spree • Sniper Spree • Splatter Spree • Open Season • Slice 'N Dice • Sharpshooter • Vehicular Manslaughter
Consecutive Kills	Double Kill • Triple Kill • Overkill • Killtacular • Killtrocity • Killimanjaro • Killtastrophe • Killpocalypse • Killionaire
Special Kills/Vehicular Medals	Beat Down • Assassin • Sniper Kill • Grenade Stick • Laser Kill • Oddball Kill • Flag Kill • Incineration • Killjoy • Death from the Grave • Splatter • Highjacker • Bulltrue • Wheelman • Skyjacker
Objectives	Last Man Standing • Killed Flag Carrier • Flag Score • Killed Juggernaut • Killed VIP • Killed Bomb Carrier • Bomb Planted
Objective Sprees	Hail to the King • Infection Spree • Zombie Killing Spree • Juggernaut Spree • Mmmm Brains • Hell's Janitor • Unstoppable
Bungie.net Only	Steaktacular • Linktacular