It’s totally fine to have a bad guy as a protagonist. Anti-hero is a very common trope in media, look at shows like Breaking Bad or Sopranos. A lot of people are saying to justify her actions - which you can - but I don’t even think you necessarily need to do that. She doesn’t have to have some deep, dark backstory about why she does what she does. Some people are simply bad. What you SHOULD do, is make sure you are not glorifying her actions, and give the audience someone to root for. If you make her a sympathetic character with good qualities and storylines, especially in her “normal” life, people will root for her despite her killing. If she isn’t a good person and this also comes through in her “normal” life, that’s also okay. But you need to give the audience someone to cheer for, whether that’s the detectives working the case, a potential victim, etc.
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