Very, very big difference between running to render aid to the victims of a bomb blast, an accident, a fire, etc., and drawing your firearm and wading into a situation that otherwise did not involve you.
Unless you take into consideration that there was a risk of an assault immediately after the bombings. It's hard to say what you would do since bombings are followed by confusion and chaos. I think we are fortunate that those two didn't decide to attack the crowd. In Boston, besides the cops, who would have been armed?
Myself, I couldn't see myself passing by a dying man or woman or child, but were my family with me, it's hard to say in what order I would do things.