demented vs crazy

demented is not a noun while crazy is a noun.

Word	Noun	Adjective	Verb	Adverb
demented	No	Yes	No	No
crazy	Yes	Yes	No	No

As adjectives, demented and crazy are synonyms defined as:

Other synonyms of demented include brainsick, disturbed, mad, sick, unbalanced, unhinged.

demented (noun)	crazy (noun)
	someone deranged and possibly dangerous

demented (adjective)	crazy (adjective)
affected with madness or insanity	intensely enthusiastic about or preoccupied with
	possessed by inordinate excitement
	foolish; totally unsound
	affected with madness or insanity
	bizarre or fantastic