These acts are being perpetrated by psychopathic assholes. Many who have been inspired by a scientology like, terrible fan-fiction sect of Islam.
I don't think it's necessarily true that people who do acts like this are psychopaths. For you or me to do something like would indeed most likely indicate a severe psychotic episode, but for them, the decision to take out as many unsuspecting, unthreatening, non-dangerous people as possible (kids included) is a rational decision decision made with lots of deliberation.
The idea of martyrdom is a cultural phenomenon that exists in all societies. In the U.S. our soldiers are patriotic martyrs. To some, cops who die in the line of duty are martyrs. We just don't use that word for it. We call them "heroes." And in certain, significant circles of Islam, martyrdom/heroism consists of what happened in Nice yesterday. And if that is the case, then mental illness can't be used as an excuse.
I tend to think that approaching it from an angle that refuses to hold the religion itself responsible ignores a large part of the reality. And that reality is that what we call terrorism is something that Islam considers acts of heroism. What's the answer? Fuck if I know. Eternal vigilance?
But there is another question: what is France going to do? Surely the French cannot sit idly by and and simply say that not all Muslims are bad so nothing can be done. And that's the problem. Something has to be done. Somewhere there were Muslims who took no part in this latest attack, knew it was going to happen, yet said nothing. And it's those people who need to be convinced that this is not acceptable. Easier said than done though.
