Why does my dog try to kill (or help) my kitten?

I have a kitten, and have HAD kittens before, that my female, fixed, 11 year old dog has tried to kill. I'm not sure if she is actually TRYING to kill the kittens, or she thinks she's helping them. My first set of kittens were just old enough to stop nursing off their stray mum, and it was winter, so I decided to slowly take away the kittens as not to make her panic. She never noticed, and the kittens were fine. They drank water and ate normal food. I left the kittens out in the living room one day, only to come home and find my dog had killed them all. She had bit the heads off. Now I witnessed her doing this with my new kitten, but all she did was try and GENTLY pick the kitten up in her teeth, and as she picked her up I caught her and made her drop it. It's unhurt and fine, but I'm not sure, does she view the kitten as a baby of her own, or possibly does she think it's an injured, orphaned animal and she wants to help? Or does she want to kill it off for sport? We have a few full grown felines and she has never tried to hurt them or anything. In fact, she'll often let one of the cats sleep next to her. She's never attacked or been attacked by our cats.