Actually, the cat was just fascinated by the way the baby deer smelled. It's intentions were neither play nor harm. If I knew for a fact it was a female cat I'd say it was trying to mommy the thing.
Interesting that the mother deer was fine with the cat until her protective instincts kicked in at the sight of the dog, then she was basically in a maternal rage. Props to kittie for holding her ground and getting in a swipe before hightailing it