Not the best example, but this should hopefully get the point across:
Lets say you have this one friend, who really hates some journalist, maybe a movie or game critic or something. He says something like:
"This guy has no idea what he's talking about! He thinks a game/movie deserves only a 5/10 because of this one thing!"
Even though the journalist gave a very reasonable list of complaints he had with a game/movie and it was not just because of "this one thing!".
What do you call such people?