The two young brothers from Cambridge seemed to be on promising paths, one a scholarship student at college, the other fighting for a national title in amateur boxing. And then, apparently with little warning, they veered violently off track, deep into the darkness, setting off deadly bombs, authorities are convinced, at one of Boston’s most iconic and joyful events. To those who knew them, the apparent transformation of Tamerlan Tsarnaev, 26, and Dzhokhar Tsarnaev, 19 — ethnic Chechens, born in the former Soviet territory now known as Kyrgyzstan and transplanted to a working-class Inman Square neighborhood — seemed almost inconceivable....