What is and isn't canon is largely up to the individual player. The Killing Floor franchise has never focused too much on the story, and it's been retconned in places as the franchise moves forward, but there are bits to follow for those who are interested.
Starting with the original mod to Unreal Tournament 2004, the story went through changes even then, and it's not clear how much (if any) of the story from that version is even true once the series was brought to Tripwire Interactive and got it's first retail release. From there the series expanded with a sequel, two different spin-offs, and a short film. There have also been artbooks and developer comments on forums and in interviews.
So What is Canon?[]
You are a survivor or mercenary fighting against waves of Specimen in various locations during an outbreak caused by Horzine's former CEO, Kevin Clamely. Only the original 5 core maps and Transit follow the story. That's it.
What's Probably Canon?[]
The first eight maps in Killing Floor, along with the Objective Mode of Transit. Followed by the non-Event maps in Killing Floor 2 that were made by Tripwire Interactive.
What's Probably Not Canon?[]
Community-made maps are a bit iffy. Some are plausible (like Mountain Pass) while others are less so (Hell). However, they are in the game so you are free to take them at face value if you so choose.
Event maps are not canon. Some aspects may have been repurposed like DARs originally being on Moonbase but even then DAR v2 from the sequel may also be a non-canon character. The events of Ringmaster Lockheart's in both games are also non-canon and is simply a fun side story.
What About the Original Mod?[]
While it had a Story Mode, there have only been a few references to it in the retail release of the first game (namely with the biographies of Lt. Masterson and Private Schnieder). However, you could probably take the Story Mode as a prologue to the first game if you squint a little. The big hiccup is what you think about the Masterson situation that it brings up.
Ultimately...[]
It's up to you. If you don't care what the story is and just want to kill some zeds, that's fine. If you want to just stick to the more believable (within the confines of the setting) version of events, that's fine too. And if you want to believe that DJ Scully killed mutated hillbillies created by toxic runoff from a Horzine factory while Mr. Foster chased an Evil Santa Claus from an evil toy factory to his icy lair and finally to the moon, and a man in a chicken suit charged into hell itself, then that's okay too.
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KILLING FLOOR MOD
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KILLING FLOOR: CALAMITY
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