"Oh my god, they killed Kenny!" is a trademark gag used in many episodes of South Park, particularly during early seasons. After Kenny is killed by some method or person, Stan or Kyle (on one occasion, Cartman) said Stan's line, "Oh my God, they killed Kenny!", with Kyle responding, "You bastard[s]!". The creators of the show, Trey Parker and Matt Stone, have said that Kyle is actually directing the insult at them, for writing Kenny's death into the show.
The phrase is so iconic it was translated into Portuguese, as seen in "Chickenlover" as well as Spanish, as seen in "W.T.F.". The line was also uttered in Farsi in "Osama bin Laden Has Farty Pants." It was later used by Stan to find Kyle in "Super Best Friends" so that he could save him from drowning and get away from all of the Blaintologists. The last time the full phrase is used is in "Make Love, Not Warcraft". However, the most recent occasion was "I Should Have Never Gone Ziplining" where a live-action version of Kenny dies out of boredom. Kyle says the line and he and Cartman got into an argument about it.
The line originated from the short titled The Spirit of Christmas: Jesus vs. Frosty, when Kenny (an early version of Cartman who was named Kenny) is thrown by Frosty and dies. To this action, Kyle reacts by shouting "Oh my God! Frosty killed Kenny!"