Is a Muslim's fear of a criminal, or jinn, or entering abandoned places considered a form of shirk (polytheism)? And does the same ruling apply to describing a person as "harmful" due to their physical actions, or describing something as "harmful to health"?

Fear of harm from evil people is natural and not blameworthy unless it leads to performing something unlawful. Natural fear of jinn is permissible, but if it leads to the corrupt belief that they can harm and benefit without Allah, then it is major shirk (definition). There is no objection to fearing entry into deserted places if the fear is natural, but if it stems from fear of secrets (supernatural hidden dangers), then it is shirk. Stating that a certain person can cause harm is not blameworthy if the harm is a known, tangible one, provided there is a belief that harm and benefit are by Allah's permission.