The deli worker accused of fatally stabbing his Harlem ex-girlfriend in a jealous rage early Monday was coolly going about his business at his Bronx job just hours before the slaying, haunting new video shows.

Luis Marquez , 41, is facing murder charges for allegedly knifing Carmen Lopez up to 50 times when he ambushed her inside her West 129th Street apartment building around 4 a.m. after lying in wait for two hours.

But leading up to the frenzied attack, footage obtained by The Post showed the suspect milling around his job at RC Meat Market in his “usual” demeanor before punching out around 7 p.m. on Sunday.

The video showed him wearing a black apron and ball cap as he’s cleaning up equipment behind the counter along with another worker.

As he left, he even told his boss he’d see

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