There should be a sub-genre in thrilling writing about the stories where somebody stumbles, accidentally, into witnessing a major crime but the perpetrator keeps countering each move with one of his or her own. The reviews on this thread keep referring to Hitchcock's REAR WINDOW, which certainly is the best known version of this plot, except for one element that is not in that film until very late. A better film to compare WITNESS TO MURDER to is actually THE WINDOW with Bobby Driscoll, Arthur Kennedy, and Paul Stewart.

Difference between WITNESS and REAR WINDOW, of course, is that in the latter film Raymond Burr is unaware of why he is the center of so much attention by the police until he sees Grace Kelly's gesture regarding his wife's ring and the only person who can see it is Jimmy Stewart across the courtyard of the apartment buildings. Then he realizes who has been tipping the cops off about him. But that is about ten minutes before the end of the film.

The situation in WITNESS TO MURDER is almost identical to THE WINDOW. Barbara Stanwyck happens to see a woman being strangled in an apartment near her's by George Sanders. But Sanders (like Paul Stewart in THE WINDOW, notices her and prepares accordingly. He (like Stewart) has nothing to hide when the police (Gary Merrill and Jesse White) show up. He is soon analyzing Stanwyck for them as a neurotic spinster who hallucinates. And he is quite convincing.

The difference between Sanders and Paul Stewart in their comparative film parts is that Stewart killed his victim in an argument over business (Sanders was in a sexual rage). Moreover, whatever one thinks of Stewart's glib and careful killer, he is not getting deeper and deeper into crime out of any political or intellectual views. Put another way: if Bobby Driscoll had not witnessed what happened, but was sound asleep (and Stewart was sure of it), Stewart would have hidden the dead body somewhere, and he and Ruth Roman would have packed up and moved to another city. Roman's loyalty to him would have reassured that there wouldn't have been any problems on that end.

But with Sanders he approaches the situation from a "spiritual" side that Stewart would have found incomprehensible. We learn (and it is a point that Merrill finds odd and troubling) that this suave, courteous, and intellectual man is a defender of Nietzche's "superman" theories (as twisted by the Nazis) and apologist for the policies of the Nazis in several books. His treatment of his initial victim, and his subtle and continuous persecution of Stanwyck are of a piece (he does not believe such inferior types should threaten him). Towards the end he even intends to make her death appear to be a suicide. Stewart felt Driscoll was a viable threat to his freedom and security, but he never has a speech suggesting the boy was a biological inferior.

WITNESS TO MURDER is a good thriller, but it is not one of the all time great ones. Still it is a worthy picture, the only one where Stanwyck and Sanders appeared in together. So I give it a "7" on the scale, recommending it as an interesting version of the hunted turned hunter genre of thriller.