'Troll' seems to be a name for someone whose opinions you disagree with. Since internet is a huge echo chamber, any sign of contrary opinion is seen as trolling (like 'Are you serious?' or 'This guy must be trolling'). Calling someone a 'troll' is a psychological defense mechanism where a person is trying to use a label (i.e. do ad hominem) instead of rethinking their beliefs, which is hard to do.