If anyone does release a raccoon (or any other creature) make sure the trap opening is pointed towards an open area. Don't do like I did once and set the trap opening in front of some large rocks. The coon felt trapped and freaked. Scared the living shirt out of me. The next time I caught a coon...
There's not enough CO2 in car exhaust to kill him, but there's definitely enough CO (carbon monoxide). That would be the most humane (like going to sleep).