"The brothers, who lived in Cambridge, came from the former Soviet Union to Massachusetts about a decade ago. They left evidence, including on the Internet, that shows them straddling two cultures, an America focused on entertainment and consumerism and a Muslim faith tradition that emphasized devotion and purity."

Wow.  We sound so shallow. 

This is an excellent article that explores some hypotheses about what might have driven these two brothers an act of extreme violence.  It's still all mystery and conjecture at this point, however. 

"A Muslim faith tradition that emphasized devotion and purity" is misleading.

These guys hurt children. No devoted Muslim would do that.

Some revelations here:

http://gawker.com/5995132/did-tamerlan-tsarnaev-pressure-his-younger-brother-into-terrorism?

It does seem to be the case that the older brother corrupted this boy, perhaps even out of jealousy... the sad part being he escaped with death and the boy will now suffer for him....

...or, the boy, being a boy, will tell us everything so we can understand what they were thinking.