With all the scientific evidence coming out about how rage is related to head injuries, it's probably possible that boxing damage and CTE could have played a role in the Boston bombings. This doesn't excuse what happened, but certainly a brain injury could trigger aggression in a person, or contribute to an aggressive pattern that already exists.
Boxers develop injuries, those injuries lead to depression and violent/stupid actions. That's true.
Blaming this on boxing injuries is stupid and frankly offensive though. With the myriad of football/hockey/boxing athletes that have suffered major brain injuries, how many have attempted to blow up as many people as they can? This was an impressionable moron that got brainwashed and did the same thing to a scared younger brother.
The man was not a professional boxer and I don't think this could be blamed on a boxing injury. From what I have read, the elder brother was very much to himself and had hatred toward America. He felt as though he didn't belong and he didn't understand us. His customs were very different than ours. Blaming it on boxing would be the same as pulling the insanity plea. He was well aware of his plan and he was an extremist.
Whether the surviving Boston bomber has brain damage from boxing is irrelevant. As a young person any damage his brain experienced was more quickly healed, and his adult executive functioning capabilities were not fully developed. Teens generally do not have the depth of person to fully understand the consequences of their behavior for the building or the destruction of the common good. When his role-modeling brother became radicalized planning these destructive atrocities, he would not have had the resources to counter his influence.