I was going to raise this point but I see you beat me to it.
There are absolutely valid reasons for preferring f/t over live but not wanting to cause suffering of the animal isn't one. If you're ordering f/t, you have caused the suffering and death of that rat just as much as throwing it live into the snake tub. You want to talk about horror stories?!!! Read about the ways people have put down rats for f/t. Top of this list is putting them in a 5 gallon bucket of water and then adding another bucket with a brick on top to drown them. Dispatch by bp is much more humane. Do you know how your f/t are euthanized, especially if coming from a local pet store?
There are ways to do CO2 euthanasia as humanely as possible. This involves the use of a regulator so you can control the flow which is necessary to induce unconsciousness without panic. Dropping a CO2 block in water is not the way to do this. I'm not speaking out against the use of CO2 blocks, but if you are using them or buying f/t from someone who does, it's no different than the fear a rat feels as it's being constricted.
There's a strong disconnect sometimes with people when they don't do the killing directly (or by bp proxy). My family is not happy with the way chickens are raised and treated in the factory system so we raise and slaughter cornish rocs ourselves. We give them as quick a death as possible (much more humane than anything gets from the grocery store) and it's much higher quality food. I still get amazed when I hear someone say "That's horrible, how could you kill a defenseless bird" or some such as they're biting into their chicken sandwich. Same thing with the live vs. f/t debate if you're bringing suffering into it. Just because you're not witnessing it doesn't mean it didn't occur.
I feed about 1k live each year with not a single problem yet. Overproduction is humanely euthanized for f/t feeders as necessary.