Sadly it will probably end up that the Louvre attacker had several screws loose
Now now...that's the typical narrative when this happens - except the same thing is said about right wing attackers who use violence and that's conveniently forgotten.
There are sadly people who have issues that probably means they use violence to justify their attacks. If you keep hearing people, respected people, people in authority, use dog whistle tactics etc - that can help make your more extreme views seem acceptable, especially if you live in an echo chamber of reinforced views. It makes racism, sexism, homophobia seem acceptable if you live in an echo chamber. And if you are told that 'other' people are attacking your views, damaging your way of life, that can trigger something.
You'll note that I did not refer to a specific religion because that can be applied to anything. Attacks on Muslims, attacks on LGBT people, women, black people, far right people, Westerners etc.
If all you hear is hate, if you all are told is that other people are not 'you' and they will destroy your way of life, if you are manipulated to think like that and if you are vulnerable to such thoughts, it's not a short leap to see how you can carry out violence against someone.
I fear it's the judges next....