All fandoms engage in gatekeeping. Whether or not we agree that contemporary media fandoms comprise something like ‘communities’, the ‘-dom’ suffix to the term ‘fan’ necessitates some conception of a group, and groups have boundaries. These boundaries are always in tension, and self-defined true crime fans are a tricky case. True crime is mainstream, an endlessly profitable and staple of the media landscape. However, true crime fans must negotiate and police a boundary that separates them from the posited figure of the Bad Fan. This article examines the Reddit subforums r/TrueCrime and r/SerialKillers, analysing posts that discuss and police the boundaries separating the right kind of interest in true crime from this posited Bad Fan. I argue that while true crime enthusiasts tend to present their gatekeeping work as an ethical practice, it is often more to do with maintaining gendered norms than it is about morality or propriety.