How long had the male been in with the rest of the females? when I have a death in a tub, I usually replace it with a young rat. With males, they tend to get more docile as they get older. Sometimes that can be a bad thing when trying to add a male to a new setup. If the male isn't aggressive enough, or if the female(s) see him as a threat, they will simply kill him. Eating him is just natural, it's not a sign of a bad rat.
I've also seen the opposite on some occasions... Sometimes adding a new male to the tub, he will be the aggressor, but they usually don't kill the females, just beat them up. That doesn't happen very often.
Or, he may have just died.... It happens. Eating the dead rat is just extra protein, any rat will do it...
I know alot of people like to seperate the females as they get pregnant, and I even tried doing it for a while myself... but, I find that keeping a breeding group together is less stressfull on all of them... Not everyone will agree, but I just see it as, the females have to "re-adjust" when you put them back in the mix.