human beings vs mankind

human beings and mankind both are nouns.

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

As nouns, human beings and mankind are synonyms defined as:

human beings and mankind: all of the living human inhabitants of the earth

Other synonyms of human beings include human race, humanity, humankind, humans, man, world.

human beings (noun)	mankind (noun)
all of the living human inhabitants of the earth	all of the living human inhabitants of the earth