the thing about freezing is it slowly rips apart the cell structure as it works, think about how meat burns in the freezer... and try and decide if you'd want to die like that.
besides the pain factor, it's not instant at all and being instant is the only way to be humane. somewhere, there's an ASPCA (or someone? i forget who) official list of methods of putting animals down, i referred to it when i had to put down a very sick frog and freezing (for any animal!) was strongly discouraged...