PARIS — Early on the morning of Sept. 11, a 67-year-old jeweler in Nice, France, raised his unlicensed semiautomatic pistol and shot a teenager in the back as he and an accomplice sped away on a motor scooter with gems they had stolen at gunpoint, prosecutors said. The body of the youth, Anthony Asli, 19, was discovered on a nearby street lined with small grocery markets; the jeweler, Stephan Turk, was arrested last week and charged with murder.

The case has set off a national debate in France about how to define self-defense, and about the decision to charge Mr. Turk. Many have rallied behind the jeweler, lauding him as a hero who was protecting his life and property, while others have accused him of being a reckless and unfeeling vigilante.

The far-right politician Jean-Marie Le Pen said he “would have done the same” as Mr. Turk, and a Facebook page created to support Mr. Turk by an anonymous author has attracted about 1.6 million “likes” — a figure so high that some social-network experts have challenged its veracity.

At first Mr. Turk was held in police custody, but after a public outcry, he was moved to an undisclosed location under house arrest, wearing an ankle monitor. Mr. Asli’s sister, Alexandra Asli, said Mr. Turk deserved prison for shooting her brother in the back. Mr. Asli’s father said his son had been shot “like a pigeon.”