The Second of May 1808 depicts the beginning of the uprising when the Mamelukes of the French Imperial Guard are ordered to charge and subdue the rioting citizens. The crowd sees the Mamelukes as Moors, provoking an angry response. Instead of dispersing, the crowd turned on the charging Mamelukes, resulting in a ferocious melee.
Goya was probably not present during the actual Charge of the Mamelukes. His supposed presence was first suggested in a book published 40 years after his death, reporting on conversations the author claimed to have had with Goya's gardener. His paintings were commissioned in 1814, after the expulsion of Napoleon's army from Spain, by the council governing Spain until the return of Ferdinand VII. He chose to portray the citizens of Madrid as unknown heroes using the crudest of weapons, such as knives, to attack a professional, occupying army. That did not please the king when he returned, so the paintings were not hung publicly until many years (and governments) later.
Goya chose not to paint any single action or to have any single focal point to emphasize the chaos of the drama.