Any fangirl/boy who purports to be a fan but who is actually engaged in dissing down, covertly or overtly, the object of a fandom, often for hidden agendas of their own; any fan who approaches their fan subject from the viewpoint of hiding their criticism under a false facade of appreciation; anyone offering veiled criticism as 'friendly' advice; anyone offering unwanted critiques, analysis or opinions which are thinly or poorly-veiled attacks or backstabbing. Anti-fans often purposefully start flame wars. Key style of an anti-fan is to appear reasonable and concerned at all times, often quoting dubious "sources" in their arguments.
Some alleged fans on the Britney Spears discussion board are more in the nature of anti-fans, with barely concealed gloating over her rehab - "I'm a big fan and feel really sorry for the girl, but she's losing it. She craves the attention but then comes crying when the paparazzi hound her. I wish I had photographers chasing me. LOL!"