Referring to the movie, since I haven't read the graphic novel*:
The government imprisoned people without charge, tortured them, used them for unethical human experiements that killed almost all their subjects, and them released biological weapons upon a civilian population in the hopes that the resulting fear would allow them to cement their hold on power.
V killed people who had been directly involved in the above crimes, who could not be legally tried because of government corruption, and made a TV appearance urging people to resist government opression. He also blew up an empty buiding, after giving fair warning a year in advance.
Who is the terrorist here?
*I do intend to, but every time I go book shopping I keep forgetting to check for it.