It’s always interesting when crime dramas turn on their recurring characters, such as what happened in "Secrets Exhumed," this week’s episode of Law and Order: SVU, when it was revealed that Marcia Gay Harden’s FBI agent, Dana Lewis, actually killed a woman in a jealous rage 25 years earlier. Rarely does a show have the guts to make a main cast member into a killer (Bones was an exception to this rule, and people are still mourning the loss of Zach Addy), so it’s usually a lab tech or a cop or someone who people know, but don’t necessarily care all that much about, who ends up as a murderer.
I suppose you could look at this two ways. If you’re an optimist, it’s a brilliant story-telling technique that forces the viewer to identify with a villain more than they would if it was just another killer off the streets. It creates grey areas where issues can be explored and beliefs might be challenged.