Reff: V for Vendetta vs Graphic Novel
The movie takes the path of creating a perfect villain, which is the government and a hero who saves the people from their government.
The movie makes the Norsefire extremely evil by introducing the element of creating a virus and attacking and killing thousands of their own people just for the sake of power. The book doesn't do that, yes they are evil and corrupt but gained their power during the period of the nuclear war.
The book makes V truly an idea as Evey takes on the mantle of being V and continues the rebellion. The movie shows more of a good vs evil battle where even the good fights fire with fire and dies winning. Evey is not shown taking the idea of V forward. V is one person who has died.
That does change the theme out a bit but I suppose it was good for the movie given it needed to be packed into 2hrs.