That's pretty cool... I don't know how far to take it without getting too cheesy. Blood-red mottling? A reaching zombie arm for the clasp or drop loop? Guts instead of line? OK, maybe I do know
If it were you, personally, I'd go for subtlety. Start with the colors. Hope you got Muddy or Jungle Green with green & black G10 or black canvas Micarta.
Next, you know how inlays are exposed through a "window" of an overlaid piece of leather? Try that, but cut that window into the shape of a bite wound or something. A cool inlay might be something like ray skin--lots of small bumps. I'm not sure scratch marks would shout "ZOMBIE!" clearly enough, but you could try scratch mark-shaped overlay "windows" too. I just feel like that would work better with tooling and a bite wound would work better with inlays. Imagine a dark, bloody red ray skin?
How would you feel about breaking up those clean edges on the sheath? I'm no leather pro, but perhaps there's a way to make it fuzzy and ragged without compromising the build quality. Maybe for the superficial layer of leather, something more dull and rough could be used, like latigo or suede. Those should just be the outermost layer though because I think those can hold more moisture than the type of leather that's usually used in sheaths, so it could harm the knife.
Another take on the bite wound... what if you just left a bite wound-shaped hole or two open in the sheath, making parts of the knife visible? You know how some zombies are missing their cheeks or lips and you can see their teeth, and it looks pretty unsettling? Or Harvey Dent/Two-Face from The Dark Knight? I was thinking of something like that. Just a sort of "exposing the insides" theme.