The one thing many of the previous comments - that try to explain the deer’s behavior, don’t explain is that when approached by another deer it behaved like a predator, chasing competitors away. If it thought its behavior was “defensive” against a predator, you’d think the deer would have welcomed other deer as supporters. Instead, the deer wanted IT’S PREY for itself. Is that/would that be typical of deer behavior?
https://www.outdoorhub.com/stories/2013/09/09/the-meat-eating-habits-of-deer/
Do not go looking for “deer eats bird” videos.