Don't be comforted. Sacrificing mice is still extremely cruel and is the reason I specifically don't work with mice.

Animal protocols require you to use 2 methods of killing to ensure the mouse is dead, and I'm assuming the decapitator is meant to be used as the second method to ensure death.

The first method of killing often involves gassing mice with CO2. This takes several minutes, and the mice definitely know that they're getting gassed. After some time, they start panicking and scurry around the chamber looking for a way to escape. After a bit more time, they get weak and collapse. This is typically when you do the second method of killing. Some protocols require you to stab the heart with a needle and drain the blood. Some require you to snap their neck (like in the movies, except what the movies don't show is that the mouse flails and twitches when you do it). Some require you to inject with ketamine (ok, this one isn't so bad). And I guess now there's a decapitator that lets you just cut the entire head off.

Even among researchers, there's a pretty sizeable number of people who don't want to work with mice