Mice smell horrid.
I had a 1.1 pair in the past and boy, they reeked a few hours after a fresh cage clean compared to an entire rack of rats who haven't been cleaned in 6 days.
Rats>>>>>>>>>>>>>>>>>Mice
Anyway. In regards to mice specifically, I have no remorse or sad emotion for them when they pass. I don't feed mice to my snakes, because I prefer that they eat rats. But I occasionally feed mice to my ferrets and dogs.
And just to note that a snake coil is no worse than a co2 chamber. A snake coil stimulates endorphins, which do dull the pain receptors. And in both methods, the feeder is dead in about the same amount of time if you are feeding the appropriate sized prey.
And I suppose it doesn't help that once you've euthanized hundreds of rodents yourself or watched your predatory pets kill their prey, you unfortunately become rather desensitized.
Don't get me wrong, I love my rats and enjoy keeping them. I do respect & appreciate their lives. But I no longer get sad from feeding them off like I did when I first started keeping carnivorous animals.