The main reason people keep making this is assumption is because of a couple of surface level things and some less surface level things
- both wear a mask and V is one of the few movies we see someone wearing a mask and killing people who doesnt look grotesque
- the voice does bare some similarity but thats largely based on the Voice modulation to simulate the mask
- both share a background in the arts. after V's escape he went on to appreciate old films/music/poetry/paintings
- both have a lot of self respect at least on a surface level and while both men are monsters (even if V's cause was just) they choose to carry themselves with class.
however despite the similarities i dont feel they are the same in fact i see them as opposites because V doesnt necessarily feel trapped he is very aware of what he is and openly embraces death after his task is done, Jhin on the otherhand is only vaguely aware of what he really is and is so trapped by his persona and his compulsion that he can't escape himself even in his most lucid of moments, he also is entirely selfish as he only furthers his own pleasure by killing.