Is hostility a mental illness?

Can be a symptom. Hostility isn't a mental illness, but it certainly can be a symptom that is a part of a number of mental health disorders.

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Is hostility a mental illness of sorts please?

Hostility in and of. Itself, is not a mental illness, but as a symptom, it could be related to a number of possible illnesses. I would strongly advise first a thorough physical examination with your family doctor to run out any underlying medical problems as the cause and then an evaluation with a psychiatrist. We are here to help.