Oh Red, I'm so sorry this happened to you.
My very very loved and missed indoor cat got out a bit less then a year ago and was hit by a car. He was hit on his way home from his adventures. Even though it still hurts, never once did I blame the person who hit him - he was a black cat and it was on a curve.
So, I want to tell you thank you for stopping. Even though you knew what you would find, and were unable to locate the owner. You still did the right thing. In no way was what happened your fault. Those images will stay with you for a while, the best way to get rid of it is to watch TV (not something I do often) and blot it out. Watch something engaging enough to distract your brain.