Maybe Boston looked better than Chechnya? Boston has a history of supporting the IRA. Could that have made him think he would get more sympathy for extreme views than it turned out. There are endless possibilities.
Boston is home to MIT and Harvard Business School, there will have been many runners from around the world and all of them will have been much more privileged than most of us. To a pair of refugee lads that may have been a personal affront.
I think I would boil it down to a broken family with developmental trauma. Hate and psychosis. It is better than most of the other options