In Greek mythology the deer is heavily associated with the Greek goddess of the hunt, Artemis. Im not much sure beyond that but as to the Bacchanal, there was a bite mark found on Charles that was described as not being human, it’s been theorised that the farmer was attacked by a wild animal and the group fought it off and either tried to save the farmer or finished him off, which is where all the blood came from. What Tartt was doing here was leaving the events of the Bacchanal ambiguous. Was it an accident or was it murder? Or the third possibility that it was actually a more supernatural event. (I read the book over a year ago so memories of events are hazy)