If I whack a mouse on something to kill it and then freeze it will the mouse look like its bleeding internally when I defrost it?

Most of the time yes at least in our experience.

In lieu of a CO2 outfit to kill mine with, If I need to kill off several to freeze I put them in a quart sized glad freezer bag, the ones with the double zips, push out all the air and seal it shut. They'll fall asleep from lack of oxygen in a matter of minutes. Then I spread them out flat in the bag so nobody gets frozen to anybody else and stick em in the freezer. Now rats might be a different story.

Quig

I think it depends how you kill it.

If you smash it against something than yeah, most likely.

I pinch their necks. Takes them out quickly and, at least I hope, humanely. When they are defrosted they dont show any damage. At least thats been my experience. Your mileage may vary.

I don't whack mice either. If I have a bunch to kill, I use the CO2, otherwise I just lay some tweezers or something across their neck and pull their tail. It's a very fast death and not messy at all.
Doesn't work with rats, they have those body builder necks and you just pull the tail off without killing them. All the rats get the CO2 unless I'm just killing a few to be fed immediately.