mid-15c., "deadly, depriving of life," present-participle adjective from kill (v.). Meaning "overpowering, fascinating, attractive" is 1630s, from the verb in a figurative sense "overwhelm (someone) by strong effect on the mind or senses." Meaning "very powerful in effect, exceedingly severe, so as to (almost) kill one" is from 1844. Related: Killingly.
"act of slaying, "mid-15c., verbal noun from kill (v.). Meaning "large profit" is from 1886, American English slang.