What are some tell-tale signs someone is a serial killer?

Upon first meeting them

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Magenta · F

We can not assume there are any or we can make that kind of a judgement call when we first meet someone. If it were that easy they wouldn't be a successful serial killer because we or the potential victims would all read the signs beforehand and know that's what they were, lol. Derp.
Sometimes they are successful and charming or your next door neighbor that you thought was just the nicest guy.