If I had a rodent with a sack full of infected disgustingness I wouldn't have even attempted to save him. Whack! Next male goes in with the girls LOL.
I used to worry about losing good genes when I would whack a biter, but I have so many high quality breeders now that it doesn't really matter anymore.
Culling in the beginning seems like you're running in circles. Producing some rats and culling over 75% of them every generation. However, after a few generations you will have found that you did a lot of good for your colony.