Freezing has been widely shown to be an inhumane way of euthansia. It's interesting that freezing seems like common practice in the U.S, here in the U.K it is very much frowned upon. There are two main methods of euthansia which are widely deemed acceptable and they are Co2 which can be purchased at diy stores etc. and breaking the neck of the animal (This method is only really used on young rodents).