말다 means not doing it after all, so this is the 'if not, forget it' half of a pair. 아니면 말고 and 안 되면 말고 are the fixed phrases, delivered with a shrug: 얻어 걸리면 땡큐고 아니면 말고 (if it lands, great; if not, whatever), 가면 가고 말면 말고. 아니면 말고 식의 기사 describes reporting that throws claims out without caring whether they hold up, so the phrase can be an accusation as well as a mood.