"Where's thy conscience now?"

Act I, Scene 4, Shakespeare's Richard III

The two murderers dispatched by Richard to kill his brother Clarence are having second thoughts...

From a 1983 BBC television production, with Paul Jesson as George, Duke of Clarence, and Bernard Hill as First Murderer. The actor who played the Second Murderer is not named on the YouTube page, but screenonline lists him as Derek Fuke. The play's director was Jane Powell. The playwright we know.