If there's one thing we want to believe about the Boston bombing, it's that someone saw the perpetrator. Somewhere, inside an iPhone or on a memory chip, there's an image of the terrorist(s). The video would serve as evidence at a trial, and it would calm the queasy feeling that grows as the hours pass between tragedy and arrest. Someone can't just plant bombs among hundreds of people and walk away without being spotted. Not in today's surveillance/sousveillance society. Right?
But they are not real cops. They are well-meaning people who have not considered the moral weight of what they're doing.* This is vigilantism, and it's only the illusion that what we do online is not as significant as what we do offline that allows this to go on. Imagine if people were standing around in Boston pointing fingers at people in photographs and (roughly) accusing them of terrorism.