Correlations are indicative of possible causes, but not definitive. In the modern way, we now have a torrent of information about the perpetrator of the Nice massacre. The quality of that information varies considerably.
Neighbours have spoken variously of him being:
a normal person, just like others in the neighbourhood, and doing normal things like drinking and going to parties;
a weird person; who looked odd, would not hold doors open for others, and was asocial and a bit scary.
Conflicting reports of this sort are pretty usual. Ordinary social behaviour has some common ground rules, of which non-involvement is a large part. Many people do not know much about their neighbours. In ghettoes like the one in Nice there are incentives to say nothing, and to have seen nothing. Interviews off camera are probably more informative, free from possible community retaliation.
His family have said that he had a long psychiatric history including treatment for schizophrenia. Other family members have said that he had recently sent them £84,000 which is a substantial sum for a divorced truck driver with an estranged family to keep.
His mobile phone has revealed a string of female and male sexual contacts, with many admiring selfie photos. His main homosexual lover, a man in his 70s, has been identified. The perpetrator’s texts have led Police to a suspected small arms provider and to other contacts.
Video surveillance from street cameras showed that he practiced taking his truck down the route he used for his rampage, and photographed the route. He hired the truck some days earlier, and also acquired weapons. Other witnesses said they had seen the truck, being driven erratically half an hour before the attack. Apparently he sent some text messages in that half hour of waiting. He gained entry through the police cordon by claiming to be delivering ice-cream. Overall, it was not a spur of the moment act, but carefully considered.
At this point some commentators are tempted to jump to conclusions: conflicted gay murderer; schizophrenic murderer (5 times more likely to be violent than non-schizophrenics); both gay and schizophrenic; depressed after his divorce; not depressed but in a whirl of sexual promiscuity, or polymorphous perversity as Freud called it; suicide by cops; or “very rapidly radicalised” as a French government minister surmised, which recent reports put at a mere 8 days. All possibly true.
However, the general distal explanations do not really show why he committed mass murder. They fit better with a lonely suicide bid (the depression angle) or a deranged random stabbing of passers-by (the psychotic episode angle), not a planned mass murder. The particular method he used involved him seeing the people he murdered at first hand, and feeling the truck rise and fall as he went over their bodies. It was close and personal and protracted.
In trying to find out why he murdered we have a long list of possible distal causes: North African genes, Tunisian culture, previous criminal behaviour, low ability, confused sexuality and particularly conflicted homosexuality, and persistent propaganda from Jihadists about the need to attack infidels. More recently he had a divorce from his wife (more than a year ago I think) and then most recently whatever was happening on Jihadist websites or contacts or friendship groups. He has been claimed by ISIL as one of their soldiers, and this is a reliable guide, according to those who study these matters. However, I would not bank on this unless a real link is proved in later investigations. (In a sense their real claim is: “We inspire people to violence, without knowing who they are”.)
The act was terrorism on a large scale, and each distal cause increased the probability that a proximate cause would lead him to his performance: attacking French civilians on their national day. Whatever the personal shortcomings of the perpetrator, he terrorised France, which is what the Jihadist cause required him to do. Their aim is to show Europeans they are unwelcome in their own countries.
Yesterday a 17 year old Afghan refugee to Germany, given government support and fostered by a German family for two weeks, has attacked passengers in a train with an axe and a knife, leaving a blood-splattered carriage with three very seriously injured passengers and 14 in a state of shock. He had shouted Allahu akbar" ("God is great"), a witness said. In his bedroom Police have found a home-made Islamic State flag.
ISIL has also claimed this attack: "The perpetrator of the stabbing attack in Germany was one of the fighters of the Islamic State and carried out the operation in answer to the calls to target the countries of the coalition fighting the Islamic State”. Apparently this is a more “arm’s length” endorsement.
Distal causes: Afghan genes and culture and (the Facebook rest will be filled in quickly, I presume, his website history will take a bit longer). Proximate cause: ISIL?
The perpetrators have a common theme: they have been convinced that they should kill the citizens of the countries which took them in. The most obvious ultimate cause of their violence is The Cause they espouse.