hell on earth vs hell

hell on earth and hell both are nouns.

Word	Noun	Adjective	Verb	Adverb
hell on earth	Yes	No	No	No
hell	Yes	No	No	No

As nouns, hell on earth and hell are synonyms defined as:

Other synonyms of hell on earth include hellhole, inferno, snake pit, the pits.

hell on earth (noun)	hell (noun)
any place of pain and turmoil	noisy and unrestrained mischief
	a cause of difficulty and suffering
	any place of pain and turmoil