Karma Charger is a meme exclusive to the game Left 4 Dead 2. It was popularized by machinima maker and "Left 4 Dead 2 Funny Moments of Getting Owned" series director Criken.

This meme's history can be traced back to when the Charger was introduced in Left 4 Dead 2. This special infected was notorious for being spawned by the Director, attacking lone players, and pummelling them to death or charging them off cliffs/buildings. The term first appeared in YouTube user gNatFreak's video ""https://www.youtube.com/watch?v=aPTnbm1d0jU">Expert Demo Fails 2". From there the term reappeared in several videos, including ""https://www.youtube.com/watch?v=0Evdk7U0Or8">Why Modded Servers Scare Me".
Then Criken picked it up and used it in his "Left 4 Dead 2 Funny Moments of Getting Owned" series and from there it was launched into memedome. In these series, he invented the catchphrase "Karma Charger is always watching".

In-game, a 'karma charge' is done when a Charger (either AI controlled or human controlled in Versus or Scavenge) charges someone and pummels him or her to death without the survivor's teammates helping. Sometimes when someone in the team acts like an idiot but then gets karma charged, the teammates will normally laugh and leave the idiot die to death.
However, a 'karma kill' cannot be avoided without some experience. This is a popular technique used mostly by human controlled Chargers (since the AI Chargers weren't programmed to do this) in maps situated in high places, such as in No Mercy's finale and Dead Center's first map when the Chargers charges someone out of the building, leaving them helpless to instantly die. This can be avoided by dodging, which is quite hard.