Dogs do sometimes eat cats and kill them.
However not all dogs are mean to cats or will kill or eat the cats.
Some dogs and cats will actually get along really well and will become good friends and protect them.
I had a small dog that actually helped take care of the baby kittens that my mother cat had.
The dog actually got into the box with the mother cat and her babies to help her take care of them.
The mother cat got along really well with the dog and trusted her to be in the box with her and the baby kittens.
Also when the mother cat would get out of the box and take a break from the kittens the dog would go into the box and help keep them protected.
So not all dogs will eat or kill cats and some dogs will actually love and protect cats.