The point is not arguing whether Nahel Merzouk was breaking the law, or whether he was a danger, or whether the shooting was legal.
The point is that the law that made the shooting legal may be sour. It puts a lot of power over life and death in the hands of law enforcement. It's also new for France; the authority to shoot drivers when the officers are in no imminent danger is only a power they have as of 2017.
The Bastille was stormed on the rumor that Louis the XVI was going to arrest the National Assembly, not on action taken. Just as these riots are fueled on the risk of what on-paper authority has been given to French police: if they decide you could be a risk to other people and you move your car one inch, your life is legally forfeit (their call, and you'll be too dead to offer counter-testimony). People believe that bargain of safety vs. their right to life is untenable, and don't believe the system is set up to allow them to change it legally. Hence, the riots.
Earlier, you said
> The people of France, and everywhere else, want the police to stop criminals from joyriding through their neighborhoods.
No disagreement, but I think the concern is the people of France don't want so much risk to their own right to life to protect the community's right to life. Letting the cops decide whether "You probably need killing; you look like a threat" is a huge power to hand to them, especially through the lens of out-groups vs. in-groups and subconscious assessments ("I have this power to protect the people from terrorists; this guy looks like a terrorist, therefore...").