wajibul qatl

3 types of people (muslims) becomes wajibul qatl. --a married adultrer, a unlawful killer and the one who has left deen. I need to know who can be considered as has left deen. Does it mean he becomes a christian, a jew, a hindu or still believes in Allah and everything in Islam but does not follow anything without any compulsion and so on. Please respond with Quran and Hadith.
 
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