The way an assassin kills things wouldn't fit well with zombies, I think.
I do like the story, though.
"Stealth Kills" wouldn't be an effective way to deal with a zombie. Maybe cutting its head off with a cutlass, but not ramming a hidden blade through its stomach.
Also, it'd suffer the same fate as Red Dead Redemption:
"Hey man, have you played Red Dead Redemption?"
"No man, but uh... didn't they make, like, a zombie version of that? I played that. Such a cool concept, killing zombies in a western."
People pay almost no mind to the original game and instead get hard-ons over the expansion that includes zombies.