I think this situation is a fantastic opportunity for role playing and shaping the campaign, in fact it might be a pivotal moment for the group.
When I come to a time like this as a GM I like to tell the group "hey everyone, this is a big decision, and it's something that will have major repercussions on the game," and then I let them decide what they want to do with that in mind. The story can then flow from that.
Some of the big themes in Dresden are second chances, loss and eventual redemption or damnation. Heck, Harry works to personally redeem a fallen angel, so strong are his opinions on the matter. Your group chose the other path, which has just as much potential for trouble and interesting conflict!
In effect what this group has done is to say "when you make a choice, you must live (or die) with the consequences." That's a perfectly acceptable answer, but it's also one that when taken to it's logical conclusions in story terms leaves no room for error.
Characters in the Dresdenverse (and in every RPG I've ever played for that manner) make tons of bad calls and poor decisions, and spend a lot of their time trying to escape from or make good out of those decisions. Each time the group does this, everyone who's involved with the current situation can bring that to bear. A Warden facing the group down on charges of breaking a law of magic might say "why should I let this slip? You had no compassion when it was in your power to do so..."
Beyond that, a Knight of the Cross has a powerful personal story of where Peter denied Jesus _three times_ to save his own skin, and was forgiven. What does it say about their own ability to give someone a second chance?
I'm kind of drifting here, but my point is that your group made a choice, and that choice can lead you to a lot of fantastic stories, all without having to bring the heavy hand of the GM down on them. Perhaps when the game is over they'll still think they did the right thing, perhaps not, but you'll have something to talk about.
And for the record, I think all of the characters they're talking about would have done something to make sure the girl didn't die, but that's just my take on it.
--Steve