It appears that the elder brother got disillusioned/pissed-off (I guess he came here in his late teens, so didn't get the chance to adjust). He got angry. Maybe he felt that he belonged in the Olympic team and didn't make it. Whatever. But this provided the kindling; and some religious nut turned this anger into a weapon.

The younger brother was just too much under the influence of the elder, it appears from what I've read. Maybe he looked up to him, and just followed him blindly. Who knows.

As long as we keep figuring out ways to hate each other, we'll keep figuring out ways to kill each other.