This is so wrong I don't know where to start! All of this kind of work is highly scrutinized, you can check it out by starting your own work on reading up on IACUC.

Here are the accepted methods for "dispatching the mice": https://animal.research.uiowa.edu/iacuc-guidelines-euthanasi...

You can decide whether you think this is satisfying to you, or you can consult the bioethicists that helped develop these protocols.