Killing Floor originally existed as a mod before being updated to a full release. The full release brought over four of the five maps, only leaving out KF-Sewers (which had been removed after early releases of the mod).
The original release of the mod also featured a Story Mode where you played as a lone soldier in the outbreak. You started in a version of West London, then went to the Offices, then the Sewers, then the Biotics Lab, and finally ended up in the lower levels of the Manor map.
Maps in the mod only featured a single Trader location, along with a few changes to layout from the full release version.