What does Boxing Day have to do with boxing?
late 15c., agent noun from kill (v.). But a surname, Ric[hard] Le Kyller is attested from 1288. Figurative use from 1550s. Meaning "impressive person or thing" is by 1900 (as an adjective, 1979); reduplicated form killer-diller attested by 1938. Killer whale is from 1725; killer instinct is attested from 1931, originally in boxing.
(also killer-diller): not only was it a killer version/ The killer idea of the century is about to be laid on you (1970s+)noun