The town inhabitants were preparing to celebrate the Nazi capitulation to Western European forces of the previous night. The Algerians had always passively resented their French occupants. Right before the anti-Nazi celebration, the French sent the leader of the P.P.A., Messali Hadj, to jail. The Muslim population of Setif wanted the anti-Nazi celebration to become a strong suggestion that the French leave Algeria to the Algerians. Actually there was no such important rational plan.
All people are hungry, wanting.
Hungry people do not act by rational plans, but by instinct. Instead of fucking him up the ass, the cop shot the beautiful Algerian boy in the stomach. People act in accordance with the energy levels of their situations. The Muslims jumped the Europeans. Anger was out on the streets.