Apparently "gently caress" is too intense for a zombie outbreak.

If a friend got killed by a zombie, I would not hesitate to call it a "hawt boi!", "cocksucker!", "---!" or similar. I don't think I'd go like "you effing zombie!" or "f.. crap!"
So I'd say use any swear word but not like a 20-word long sentence with only swear words! xD Or anything racial or too offensive.
I do love how Bill says "motherhumper"! xD Must be the most offensive word in L4D, hehe.