mankind vs human being

mankind and human being both are nouns.

Word	Noun	Adjective	Verb	Adverb
mankind	Yes	No	No	No
human being	Yes	No	No	No

As nouns, human being is a hypernym of mankind; that is, human being is a word with a broader meaning than mankind:

mankind: all of the living human inhabitants of the earth
human being: any living or extinct member of the family Hominidae characterized by superior intelligence, articulate speech, and erect carriage

Other hypernyms of mankind include group, grouping, homo, human, man.

mankind (noun)	human being (noun)
all of the living human inhabitants of the earth	any living or extinct member of the family Hominidae characterized by superior intelligence, articulate speech, and erect carriage