mankind vs human beings

mankind and human beings both are nouns.

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

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

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

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

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