We don’t yet know the motivation(s) of the brothers involved in the Boston bombing. There is history that Tamerlan Tsarnaev, the older brother, was an isolated and embittered young man. We don’t know when or why that presumably happened. We don’t really know if this is true, as there have been contradictory statements about him.
He reportedly returned to Chechna and Dagestan last year, after living in the U.S. for many years. Both regions have militant separatist movements, and a history of having been at war with Russia. There are now reports that Tamerlan Tsarnaev returned to his homeland because of disenchantment with life in America. There is speculation that while there, he may have become “radicalized.”