The Crime of Monsieur Lange

A man and a woman arrive in a cafe-hotel near the Belgian frontier. The customers recognize the man from the police's description: his name is Amedee Lange, and he murdered somebody in Paris. Lange was an employee in a printing works. His boss was a real bastard, swindling every one, seducing female workers... One day he fled to avoid facing his creditors, and the workers set up a cooperative to go on working. What then made Lange a killer?