Autopsy of a Criminal (1963)

Carlos is a card player and extreme debtor who has sold his girlfriend's precious necklace to pay debts . However , his fiancee called Nelsie wants to retrieve it to give it to another very wealthy suitor who wishes to marry her . Then Carlos goes to the home of the necklace's buyer, but when he enters the flat , he finds the latter killed along with his wife . Who's the killer ?