Cat ate her kittens

My cat just had kittens.. there were 6 living and this morning there’s only 2 left.. I know she’s eating them because one of the kittens is missing the lower half of its body. Is this normal? Why did she do this? Should I take the 2 that’s are still alive away from her?

ETA: she is very properly cared for.. she’s an inside cat who occasionally goes out but we thought originally she was a boy which is why we didn’t get her fixed. Her food and water bowl are filled 4 times a day. Since she’s had the kittens, I put fresh water in her bowl everyday but she won’t eat her cat food.