killjoy vs spoilsport

killjoy and spoilsport both are nouns.

Word	Noun	Adjective	Verb	Adverb
killjoy	Yes	No	No	No
spoilsport	Yes	No	No	No

As nouns, killjoy and spoilsport are synonyms defined as:

Other synonyms of killjoy include party pooper, wet blanket.

killjoy (noun)	spoilsport (noun)
someone who spoils the pleasure of others	someone who spoils the pleasure of others