That is so sad. I don't understand how someone can see an animal suffering like that an not call animal control, or the police, or anything like that. My friend's Great Dane jumped their fence on the 4th of July because someone in their neighborhood shot off fireworks while the dog happened to be outside pottying - and was hit by a car. No one reported it, and my friend found her sweet dog on the side of the road outside, hit by a car. No one had reported anything, the dog was wearing ID tags, and barely breathing. Of course you would know if you hit a full-grown Great Dane - it is so sad to me that someone would have hit her and never reported it at all.
I'm sorry you had to see that, but I am glad you were there to call Animal Control and make the report. Maybe they could find the owners and let them know.