Actually, I used to go out and find grasshoppers to feed a couple of my rats way-back-when (they're long-since gone). The same rats also attacked a hamster, but I bet money that the hamster provoked them! They were very sweet rats, wouldn't hurt anyone, and that hamster was a mean ol' bitch! (pardon my language)
Truth be told, some rats are large enough to that if they were cornered by a cat or a cat-size dog, they could take them (not that they'd like to in the first place). Yuki's HUGE, he could take my mom's cat ANY day! (well, before he got sick, that is)
So yea, it's pretty normal. What MY question is though, how did a mouse get into the cage?!
And to everyone who said it was gross: That's just nature, and there's most certainly nothing gross about killing an animal for food. I don't mean to be rude, but it's rather irritating when people find something WE do ourselves gross when another animal does it. Just because we send our meat to a slaughter house so we don't have to see the blood ourselves doesn't mean it never existed to begin with...and truth-be-told, I like my steak a little rare