Why was the killer's face never revealed in V for Vendetta (2005)?

Remember this dialogue?

Creedy: Die! Die! Why won't you die?... Why won't you die?

V: Beneath this mask there is more than flesh. Beneath this mask there is an idea, Mr. Creedy, and ideas are bulletproof. IMDb

V's face was never important, he was represented as an Idea in film.

Even in the novels, V's true identity is a mystery and he visibly removes his mask only once during the entire story (at Surridge's request), at which point his back is to the reader and his face cannot be seen. Surridge tells him "it's beautiful", in contrast to her personal notes, where she mentions that the man in cell V was ugly. Wikipedia

So he was an idea or a symbol, not a face and both novel and movie writer deliberately do not show his face.

Do you mean the killer is V?
If so, he didn't take off his mask because there was an experimentation that was performed on him which gave him his enhanced speed, strength, agility, and intelligence also granted him a heightened level of pain tolerance. This experimentation affected his skin on the face so he doesn't want anyone to see his face.
Another possible explanation is that "it was what the director wants".

reff : V for Vendetta Explained

First of all, the character V played by Hugo Weaving has not much of a face left. Through all the experiments on him, he begins to disfigure. Once he plans his escape from Larkhill he sets off an explosion, the fire burns him completely. You can see it in the scene there, there is not much of a face left. Even if he showed it, you would not see Hugo.

His idea is to not scare the people of England but to make them free of fear. He chooses a mask which propels his proposed idea of freedom, the bulletproof idea of freedom.

If there were a prequel made, then Hugo would get to show his face in that.