I have 5 boys together, another group of 6 boys together and other then occasional fights (and who doesn't occasionally fight with their roommates?), they certainly wouldn't EAT each other alive.
I've never heard of a case where a rat would eat a LIVE rat. I know that rodents in general do "rodent burials" where they will eat the body of DEAD rodents. This is because in the wild, the body would attract predators.
Now, Syrian hamsters WILL kill others and I suppose maybe they could eat another Syrian alive in the process.
But I'm almost sure that if a rat were to eat another rat, that rat was probably already dead.
Except in cases with mothers eating their young, but as has been mentioned, the babies are ill or had something wrong with them, or culling so the litter is smaller and more manageable.