i dont see how anybody would have thought that freezing something to death would even be humane at all.. i use sodium pentobarbital at work and it does do the job, and very quick like, but the problem there is that you have licensed to use it.. also, sometimes its hard to find a vein on a reptile and this is too much stress than a reptile deserves.
to me, the only humane way of euthanasia on smaller reptiles is decapitation followed by pithing.
spooky