My cat had kittens yesterday and she took one of the kittens and put it under the bed I'm afraid she's going to kill it

If I was you I would just keep and eye on the kitten and cat and it might mean that you will have to separate them for a while.

Is she nursing the kitten that she hid? Washing it?
Is it possible that she was in the process of moving the whole litter?
If she has chosen to isolate this one particular kitten, sadly it may be because it has some defects and will not survive and she knows this.
Sometimes mother nature is cruel that way.
If she was not moving the whole litter and has truly discarded this kitten then you will need to step in and feed and stimulate it to urinate and defecate.
You will need to make sure it stays warm.
Most important would be to have mom and kittens checked by the vet.
Orphan Kittens (http://marvistavet.com/html/orphan_kittens.html)
Here is some helpful info on raising kittens.
First try to put the kitten back with mother, see if she will tend to the kitten, you shouldn't hand feed if mother is willing to do it. Being raised with their litter is how they become socialized.