Oh gosh, ITA about the Mom. There's something scary and devious about that woman.
The thing about DT is that he was just 19 - he would have to be a really cunning sleeper agent to pull off a ruse like that - or, yeah, a sociopath I suppose. TT was overtly radical. He couldn't hide it, or didn't want to hide it. It's weird that the younger, less experienced brother would be the more sophisticated at never revealing his true identity. I mean some of my students here felt like they really KNEW him, not just saw him and waved, or kicked a ball around, but really bonded with him the way college students who live in close quarters will. And no red flags, not even while stoned or drinking. So really I guess the facts are just that: he was capable of violence, he could kill children and smile while doing it and never gave the slightest hint to even those he spent most of his time with, aside from his brother.
However I don't believe that those things came from turning into a radical Muslim - I really think this is just something he had inside him that could as easily have come out if he joined a cult or became a professional hitman or something. I just don't really get the feeling, IMO, that he was actually that religious or a passionate follower of Islam. Maybe he was just happy to unleash casual violence and his brother's personal jihad was the path that opened in front of him, so why not.
It's like in Tales from the Darkside, when they would show the peaceful meadow with the tree and the sweet child's swing -- then highlight it in negative and suddenly it looked evil and terrifying.