Swampcritters don't usually eat their young, but its a natural thing for some species, and very unsettling for humans.
Ugh...but it does seem unnatural and cruel though....
Lissa, I think the cat ate one of the kittens that remained alive.
Butterfly,
This happens every day, all the time, among different species. It is all good...
Hi LissaKaye...hahaha, without even looking at your profile, I knew that was you. I mean that in a nice way :) Good to see you back!
Yes, it is normal. It is instinct to eat the dead babies and those that are not 'normal' so to speak.
It is a survival technique. Felines are not the only species that do this. Most carnivorous and omnivor species do this, even some heravores such as rabbits.
Don't panic, it's a natural instinct. Not all cats will do this, but most do.