Christians in the Middle East also use 'Alluha Akbar', but it's become an expression to strike fear.
I am in no way way condoning what this man did, but I can understand why he did it.
He came from a poor Tunisian family. The 'Arab Spring' caused all sorts of turmoil in the Middle East and this man wanted a better life, just like most people. It would appear that at this stage, he wasn't particularly religious. As a teenager, he drank alcohol, took drugs and was a bit of a bad character - not so very different from many British teenagers.
He got himself into trouble in Tunisia (where there's an outstanding conviction), found himself an illegal passage to Italy, where he became involved in criminal activity. At some stage, he was radicalised. It would have been relatively easy to do. His future seemed bleak and he would have been vulnerable to anybody who offered him some kind of future, even if it was in the afterlife.
Since leaving Tunisia, he had six years of a dysfunctional life. Nobody really cared what happened to him. Tell me, honestly, that any young men you know wouldn't have been vulnerable to propaganda in those circumstances.
I think that there needs to be a much greater understanding of what drives people to extremism before it can be defeated.