WMG / Rear Window

Thorwald's crime is that he steals jewelry from his employer

It is never explicitly revealed what Thorwald's crime was. He is a jewelry salesman and poor, his crime might have been pocketing some merch and reporting it as stolen or pretending that he never received it. That explains why he didn't denounce Lisa for stealing the ring (because he had stolen it himself), and why he makes phone calls and night trips (to sell some jewelry on the black market). Several trips per night in order to get rid of it quickly without getting caught with too much at once. He sees Lisa point to the ring, so he thinks that she knows that he stole it from his employer - that's why he asks why she didn't turn him in. The ending seems intentionally ambiguous in terms of what the crime was.

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