Done it a few times, but not roadkill. I worked in a landfill for a summer during university, saw a seagull that got covered in that expanding foam that some contractor carelessly threw out still half full. It was encased in the stuff, it looked like a big pink blob of bubblegum with a seagull head sticking out. I took care of it with a crowbar. I really didn't want to, but there was no way it would survive.
Also have finished off birds that my cats have brought home and near-killed, but then decided to toy with them for a while.
Depends on the situation (major road, or county sideroad), but I'd probably avoid running it over. Better to give it a swift bonk on the head with the tire iron, no blood splatter on the car.