lady killer vs ladies' man

lady killer and ladies' man both are nouns.

Word	Noun	Adjective	Verb	Adverb
lady killer	Yes	No	No	No
ladies' man	Yes	No	No	No

As nouns, lady killer and ladies' man are synonyms defined as:

Other synonyms of lady killer include seducer.

lady killer (noun)	ladies' man (noun)
a man who takes advantage of women	a man who takes advantage of women