I have a theory that some people aren't actually being killed. Obviously there are people who we can see are murdered (ms.bean, and all the people in the house...) but I also think that a few characters aren't being killed. A lot times the scene ends before we can actually see the person dying. Also, at the beginning of the series there was only 1 devil, but after Boone was fake murdered there are now 2. Lastly, there are theories going around that there is a 3rd red devil (when caufield got his arms sawed off and there were 2 devils, there was also another devil in the KKT house) So, my theory is that IF people are being fake murdered, the people who are then become a red devil. Also, when they found the Red Devils lair, there are lots of masks and capes, which might imply there are more than one devil. 

This is just a theory obviously.