Today's violence in Boston is shocking and tragic. The weird thing about the Internet is that it gives people a voice, often unfiltered, to react to the tragedy. You never quite know what you'll see. Sometimes, you end up dropping them off of your social networks (I'm looking at anyone who made it a political issue within the first hour). Then you get things out of left field, like the Mr. Rogers meme around showing the good in the middle of the bad.
Most of us never deal with intense violence directly, but as human beings, we're certainly affected by it. It's far more likely, however, that we deal with extraordinary loss or tragedy on a more personal level. There is a broad category of events that we inevitably deal with that are difficult to explain, and often impossible to rationalize. There are different things that different people do to deal.
At one end of the spectrum, people might use their faith to write a bad thing off as, "God's plan," or a more agnostic credit to some kind of fate. I tend to just attribute something like this to, "Bad people sometimes do bad things." It's not particularly satisfying, but I try to base as much as I can in tangible facts. For me at least, I find comfort in what might be an uncomfortable reality.
At the end of the day, comfort is ultimately what we seek. That there are bad people sucks, but I'm always amazed to see what everyone else does when something bad happens. People help each other out. The fear is ultimately trumped by love. If that doesn't give you comfort, I don't know what will.