I think perhaps you're not taking into account that the murders in Chicago are, generally, retail in scope (a couple at a time), while the casualties in Boston were wholesale in scope; they practically equaled 50% of your 400 number in one incident. That alone is reason enough for a major response. Additionally, there was absolutely NO REASON to think that, had the perps escaped capture, they would not have done the same thing again and again until they WERE captured or killed.
You are comparing murder figures in Chicago and Indy with number of injured in Boston. If we include the number injured in Indy or Chicago, I suspect you would at least triple that number, perhaps MUCH more than that. For instance, how many thugs end up shooting into a crowd and only inflict leg injuries?? I also seem to remember a shooting in Chicago recently that resulted in 4 or 5 deaths.
And you seem to believe that if the thugs who shoot people aren't captured, they will never shoot again. I think that is a false assumption.