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They're referring to "anti fans". You see it with online personalities especially; the most rabid fans (often parasocial, think online streaming) are the ones who will become the biggest detractors or anti fans.

Most people are "oh that's fun to watch ok" and then when they don't like it anymore, they get bored and forget about it entirely.

The anti-fans continue to follow it, but rabidly hate it.

Think Syndrome from Incredibles. He's always been the biggest fan.

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I rather think he or she means gamers for example, who send out death threats, because the developers introduced a new thing they don't like.
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Wasn't Selena killed by a scorned fan?
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Not quite. She was killed by someone who started out as a fan but then became one of Selena’s employees, who was then caught embezzling.
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