Agent 47

A genetically engineered killer with a barcode on the back of his neck who must struggle to fit in in a post apocalyptic world, while evading the sinister agents of Project Manticore who want to brainwash him and make him into the perfect soldier.

No, hang on, that's Dark Angel...

Agent 47 is a genetically engineered killer with a barcode on the back of his head, who must kill people. Star of the Hitman series.