Kill Them All is a collaborative mapping exercise that was first begun in December 2006 ... The objective of each level is very simple: you must kill every single living creature on the entire map and then proceed to the exit.
All of the maps were created under the following limitations:
1. No map can exceed 100 polygons.
2. Every map is a "Rebellion" level, so you lose all of your weapons at the beginning of
every level. Therefore, it's best not to try saving your ammo across levels.
3. Every map must have at least one Pattern Buffer -- and no terminals. There is no plot.
4. Every map is also an Extermination mission. You must kill everything to proceed.
5. Every map is made in vanilla Marathon Infinity; no added shapes, no physics, no scripts.
KTA levels are generally short and very fast-paced, focusing more on sheer quantity of enemies to kill rather than particularly lengthy or devious level design to wear you down over time. Therefore, your combat and survival skills will be pushed to the limit.
You must play KTA on Total Carnage. There is no plot whatsoever, and in fact there's nothing at all to experience except for the combat. Therefore, to play on an easier difficulty is to miss the point entirely.