humankind vs human beings

humankind and human beings both are nouns.

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

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

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

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

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