My cats had kittens 2 weeks ago I woke up today and she's killed one why is this and will she kill the others

Sometimes the mother can tell that there is something wrong with the kitten.
Another thing is if she feels threatened by something.

Is she a house cat?
Was there anything stressful going on around her?
Do you have any other cats or animals?

There is no guarantee she won't kill another kitten.
There are a *lot* of reasons she may have killed the kitten. Some cats have poor maternal instincts, and don't really switch to 'mother' mode, and will kill their kittens.
The kitten may have been sick. Mothers are known to cull their own kittens if they, to the mother, seem abnormal.
Also if you recently disturbed where she had her kittens/is keeping them, that sometimes can trigger her to kill kittens.

Finally, are you sure she killed the kitten, or did it die on its own?

Please consider getting your female cat spayed before she can have more litters.

She might kill the others. We had a stray cat have kittens on our covered porch. One day one was missing so I thought she was just moving them to a new home. Each time I peeked to see if she was going to be moving the others sure enough another kitten was gone. Last time I peeked in I saw her eating the last half of the last kitten! There was no blood or anything. It was horrible!

Sometimes a mother cat will stop feeding or even kill kitten for being smaller then all the other baby.