The above sentence. Apostrophe or not.
Smirk is a proper noun (it's a person).
Beware, it's Smirk. Be careful because it is the person called Smirk. Beware its smirk .
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Beware, it's Smirk. Be careful because it is the person called Smirk.
Beware its smirk. Be careful because that creature's smirk is dangerous.
Neither of these sentences seems likely to be said.