This guy has been around since the beginning of Act 2, when he killed Yuko, whereas the Fire zombies transformed as part of a mass ceremony much later. So he can't be one of them. I do agree he came about because of a contract with Fire, but he may well be Fire itself.
It's unlikely that he's Fire because
-Fire is said to be one of "Fami's" (Death's) pawns by Barem
-This Fakesaw Man is clearly guided by Fire devil (which he believes to be Justice devil) during his fight with Famine devil, even questioning if this was really the right target to go after
It shouldn't be forgotten that Yuko and the class president who killed Asa, were also contracted to Fire devil, so it has been making moves since the beginning, not just during the Chainsaw church arc. Fakesaw man could've been one of its first converts, explaining why it killed off Yuko.
Now it's possible there are multiple Fakesaw men. This one in particular seems to have been set up by Death and Fire devil to attack Famine, and then be turned into one of Death's pawns.