I've somehow got a white dwarf with the Daemon updates here (don't ask me how, i came home and my dad was reading it), and the justification for the Soul Grinder is pretty interesting on a fluff level. Essentially, when a daemon dies, it can choose to be re-born as a Soul Grinder, but stays that way until it kills a certain amount f people. If it dies, then it can chose to become a Soul Grinder again, or go back to the seething mass of daemons. However, as a Soul Grinder, it also risks losing it's personality, as the monstrosity of the machine can take over it's being, thus driving that badboy mental.