This is not as horrible a behaviour as you think, and also not that common.
What probably happened is the younger baby was sick....whether it was visible or not. My guess would be that it passed away during the night, and sensing that it was ill and a threat to it's own health, the older bigger rat tried to "get rid" of the baby and clean the environment.
Rats aren't the only species that practices this behaviour, but I'm the first to admit it's not pleasant and in fact quite frightening.
I do hope though, that you give rats a second chance!
