In Simon's presence, he stabs Berlioz. For some reason, she doesn't turn him in immediately, and thus becomes an accessory. During the police investigation, family friend and train engineer Jean Gabin covers for them, although he knows they are guilty.
Gabin and Simon promptly begin an affair, and Simon in her insidious manner tries to get Gabin to kill Ledoux for her. But, little does she realize that, for some reason, Gabin can't stop physically attacking beautiful women who fall for him. This leads to a truly depressing ending.
How I felt about it. Director Jean Renoir had the good fortune to be born the son of a famous impressionist painter. He inherited a number of paintings, which he gradually sold to finance his career as a filmmaker. His early films were unsuccessful, but he had much better results when the sound era arrived. By the late 1930s, he was a heralded director, and had three successes in a row: The Grand Illusion, likely his best movie; The Human Beast, and The Rules of the Game. These of course were French films; I have translated the titles to English.
Alas for both France and Renoir's career, the Nazis conquered Normandy. Renoir was lucky enough to reach America. There, he directed "A" list Hollywood movies that did sufficiently well that he remained in the U.S. after the war. Simone Simon also made to America in 1940 and promptly became a star, but the two never worked together again. In the 1950s, Renoir returned to France, where he finished his career.
La Bête is based on a novel by heralded Émile Zola. We learn this in the opening credits, where Zola's name is promoted in a nearly worshipful manner. Zola's anti-hero lead is what would today be called a sex offender, a potential Jack the Ripper who can't control his urge to kill women he is intimate with.
He knows he has the urges, but he blames them on his ancestors, whose sins of indulgence has cursed him, or so he believes. What he really ought to do is join a monastery to avoid pretty women, but if he did that the film would have little drama.
The irony is that the audience is led to believe that Simone Simon is the miscreant in their relationship, until Gabin suddenly becomes a nutcase. However, her guilt in accomplice to murder and attempted murder sets her up for a fall on account of the Production Code, which apparently held sway in France as well. Only Ledoux goes unpunished, aside from living an undoubtedly unhappy existence.