Continuity mistake: Dennis grabs the typewriter and throws it out of the window. However, just before the police enter the office, the typewriter is back on the table. Once they come inside, it disappears again.
Visible crew/equipment: During the opening credits, a large rectangular studio light mounted on a pole is reflected in the streetcar's window. It is unmistakably production equipment, not a streetlamp.
Other mistake: During the opening credits, a passenger on the streetcar can be seen waving directly at the camera.
Answer: In a flashback scene, it was shown that he had a violent temper, so it stands to reason that he finally killed her in a fit of rage.