dressed to kill vs dressed-up

dressed to kill and dressed-up both are adjectives.

Word	Noun	Adjective	Verb	Adverb
dressed to kill	No	Yes	No	No
dressed-up	No	Yes	No	No

As adjectives, dressed to kill and dressed-up are synonyms defined as:

Other synonyms of dressed to kill include dolled up, dressed, dressed to the nines, spiffed up, spruced up, togged up.

dressed to kill (adjective)	dressed-up (adjective)
dressed in fancy or formal clothing	dressed in fancy or formal clothing