I understand what you're saying, it's hard to not believe he talked to someone older than him at some point about this. And I can only imagine what it's like to spend your entire childhood in a war zone. In that sense, he's absolutely different from American kids his age.
That said, he was old enough to make his decisions. His decision was to go to Germany, hop a train, and hack people up. We may never know the true motive since he's dead, but the fact he did it still stands. Whether the excuse is being bullied, growing up in a terrible neighborhood or war zone and not "knowing any different", Tsarnaev saying his brother forced him, or just being a mental case, the fact remains they commit these terrible acts. For that, I have no sympathy. Their victims sure aren't shown any.
Now if he was closer to the age of, for example, the 10-12 year old Iraqi kids who were paid $10 to drop a grenade on American troops, maybe I'd feel different.
Just my $.02