Back in the dim misty recesses of memory (i.e. nearly a decade ago now) I ran a V&V campaign that was set in Boston. After the first couple of sessions of generic super-hero city stuff the players requested a greater use of actual Boston geography. So I gave it to them in spades - they caused a ton of collateral damage in Faneuil Hall, they flew through the Citgo sign, they stopped the theft of rare artifacts from the MFA, they fought animated lions from the the steps of the public library, they even had someone hijack a Duck Tours boat. It was all good clean fun, even if it veered a little dark and villianous from time to time.
As you can imagine the events at the Boston Marathon hit a chord with me because of that.
We gamers, especially those working in contemporary games, make a lot of use of violence as the story conflict. It's not necessary, but it's part of the hobbies formative period - it might not be in the dice's DNA but it's clearly the predominant environment. And sometimes that's good clean fun. Other times, well, we might want to look at a more extensive use of other conflicts to drive our stories. Romance. Mystery. Social Conflict. Exploration. My just completed Mech & Matrimony (the game of Jane Austen Romance and Giant Robot Combat) could, in retrospect, have skipped the giant robots and worked just fine with social concerns as the conflict. Hufflepuff & Ravenclaw is, for most of it, devoid or real violence but possessing a sizable threat of violence. One of my top sessions of my Star Trek game had plenty of threats of damage but none of violence as the USS Carter struggled to understand the plight of an alien species and then assist them. I plan on spending the rest of the year helping young kids learn to bash goblins and loot dungeons and I'm comfortable with that, as long as it's not all we ever do.
As for the events in Boston, I'll reiterate something I put on my Facebook page - terroist acts are attempts at being an allergen. They're designed to get society to turn its defenses on itself, to overreact, and therefore cause more damage to society than the otherwise limited terrorists could do on their own. The bombers are not monsters - they're criminals. Monsters are things that we fear. Criminals are things that we have civil strcutures to locate, neutralize and deal with in a way that keeps out culture intact. They WANT to be monsters.
Don't let them.