Back when I did watch more movies certain tropes were very off-putting but the way you describe demented sounds as if youre unsure of the definition of "demented". Demented means they act like they have dementia which is a cognitive disease.
I think you mean "retarded" as in they do stupid shit like "let's split up" or "I'll only smash the bad guy once and not keep shooting him--only to have the bad guy revive suddenly."
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