Blast Of Silence (1961)

Having been 'away' for some time, professional killer Frankie Bono returns to New York to do another job: assassinate some mid-level mobster. Although intending to avoid unnecessary 'contact' while carefully stalking his victim, Bono is recognized by an old fellow from the orphanage, whose calm and unambitious citizen's life and happy marriage contrast heavily with Bono's solitary and haunted existence. Exhausted and distracted, Bono makes another mistake, but his contract is not one to back out of.

IMDb: https://www.imdb.com/title/tt0054687