Normy,
The rider was missing a limb and there was no way that he would have survived that much blood loss. CPR would not have helped much and as stated if his injuries looked that severe they could have made the situation worse. I have no doubt that what you saw was beyond horriffic. What's sad is that many people wouldn't have done what you did, they would have simply kept driving. Don't question your actions. You did something commendable. If you're having trouble with this it wouldn't be a bad idea to seek professional help.
I watched a semi roll over 10 cars in highschool. 15 people were killed and the fire from the semi was so hot that the paint on my car bubbled nearly 300 feet from the accident. We ran out of class to see if we could help and the next 30 or so minutes are etched in my mind forever. A couple of the cars were so bad that you couldn't see anybody in them. A couple on the other hand weren't quite as bad. We couldn't get anybody out. They had the highway closed for a day after that just to clear out the wreckage and parked semis along both sides to keep prying eyes away. They went a step further and wouldn't let any news helicopters in the area (STL's surface boundaries). It's something that I'm reminded of on a frequent basis and I go out of my way to avoid that stretch of interstate even though it's been redesigned as a result of this accident.