I agree with you on the potential risks of CO residue, for lack of a better word, remaining in the prey, and I wouldnt recommend that either. I know the mechanism for how CO, He, and various d-orbital mimics kill us...ie, cyanide, I just dont agree with the CO2 killing slower since it has 2 O atoms, it doesnt make a difference if the body doesnt have a way to convert into something usuable. For another example, take ozone, O3. Pure oxygen, yet toxic in respiration.

This is getting kind of rediculous, but I guess even if it is the smallest bit, prey euthanized with any noble gases (Helium, Argon, Krypton, Neon, Xenon....Radon) would be better preserved since it wouldnt as reactive to potential contaminants in the body. Again, this is never going to make any difference, just got me in the chem mindset...though like CO, I dont recommend radon.