There are a lot of opinions .....
Someone once said put the pinkies in a plastic baggies and give it a whack on the counter.... my response was "Won't that just produce pinkie slush?" ... his response "I do it all the time, never happens" ... so I tried it... and got pinkie slush! (ewww)....
Another suggestion I've heard, and I actually do this if I plan on freezing pinks or fuzzies.... is throw them on the floor. They don't go squish (thank goodness) and they usually end up dead. One or two might be gasping, but I just put them all in the freezer at that point. I know it sounds awful, but I feel better than just freezing them. Hoppers and adults, if I don't want to do a mass CO2 euthanization, I either whack or I use dislocation (pencil behind the head holding the head down, quick jerk of the tail, and CRACK their little necks are broke). I prefer CO2 which is most humane (for older animals) but still isn't nice... no killing is nice! Up until recently I have never really tried to freeze too much.... but with my mouse population not really producing what I need anymore, I need to start freezing them in the right sizes when I have them, so that I don't have to worry about a shortage when the mice don't feel like breeding. I have scads of baby rats right now that I really OUGHT to freeze... all perfect mouse size... but they are so DANG cute! (urgh)
CO2 time tomorrow~