Here's a little appetizer that suggests an opportunity for a far more comprehensive story: Morgen Peck at Scientific American asks whether criminal trials provide any sense of relief or resolution for survivors of mass shootings. She talks to several authorities who say, yes, sometimes the legal process helps.
Peck also reports that "there are many reasons to think that survivors whose assailant dies on the scene will recover faster from the psychological wounds." In other words, immediate, street justice might be better for survivors than a protracted trial.
As I say, there is much more to be explored here. But Peck has given us an interesting first taste.