I think it should be normal to have compassion for any suffering animals or human. Sadly there are some people who will go out of their way to hit any animal in the road. The opossum probably had a home nearby that it could have gone back to and either died there or recovered.
I think it would be very hard to find it. It amazes me, living in a densely wooded area, that there is so much life out there that we don't see during the day. But if I turn on my back light on at night there are any number of critters that I might see. I don't know where they go during the day and I don't go looking for them either!
