This is so much better. It could still use some tweaking, but it's definitely hit the "awesomely horrible" point for me. You've put the focus on the SCP instead of its trappings, and this edit also got rid of a lot of the tonal problems I was having with it. A couple little points:
- Don't use 'shall,' especially in describing what the SCP does. It's not a firm enough word. "The main entrance is to remain locked at all times." "Any cat introduced from outside into a site under SCP-XXX’s influence will take on the characteristics of SCP-XXX-1."
- You're still not getting to the meat of the description fast enough in a couple instances. The mass of the SCP is far less important than the fact that it's a giant cat made of dead stuff. (There's also no need to use the future tense there.) "SCP-XXX is a mass of biological matter in the form of a large feline, often with extra limbs, eyes, mouths or other organs." The fact that it looks black is less interesting than the reason it does. "It is covered in a tar-like coating of dirt, blood and fecal matter, which gives the impression of black 'fur' from a distance." I'd also remove the sentence about its not being alive in a conventional sense; everything you say in there is obvious from the rest of the pragraph and that sentence just slows it down.
All of that said, this?
(Note: Researchers have described SCP-XXX making a “purring” sound. Tests have shown this sound does not originate with SCP-XXX, but is actually the sound of insects, most often blowflies, trapped within its mass.)
That's an upvote.