It can be tough to kill a rat by the whack method.. and yes, blood can come out and splatter on your wall!

That's why I built a Co2 chamber. I got a carbon dioxide bottle (used for paintball) a remote line (also paintball) and ran the remote from the bottle into a container with a tight fitting lid. Pop in the rats, close the lid, turn on the gas (you have to run an exhaust line out of the top of the container as well) and voila.. painlessly dead rats!