The individual (he was man) seems to fit a pattern if the information we are getting is correct. His parents left the Middle East and moved to a Western nation and lived by most accounts a good life and we're model citizens and were not extremists.
The children born and, or sometimes just raised, especially millennials, in the Western country are often very susceptible to becoming radicalized by hate preachers.
They feel they are owed everything. If they don't get it it aren't successful these preachers of hate will convince the more susceptible ones it is because they are Muslim and the nation is against them. Next thing you know, they are in Libya for a week and shortly thereafter committing atrocities.
See, a young man down the road from me on this path, sadly.