There were two forms of aggression present.
The dog was aggressive because he was predatory; preying on the boy. He stalked the boy from behind the car and then as he approached.
Tara was highly aggressive in a good cause; to defend her human companion who she related to as her kitten offspring (I believe).
Gail Fisher, a dog behaviour expert says that the dog’s predatory behaviour was for the purpose of moving closer to prey – as she puts it “decreased social distance” – while Tara’s aggression had the opposite objective: to “increase social distance”, to push the dog away and chase him off.
Gail also makes the point that predation is silent or as silent as possible while noise such as hissing and growling, accompanies aggression of the sort displayed by Tara.
Tara’s attitude and behaviour was highly committed and as Gail says there was could have been no doubt in the dog’s mind that “he needed to exit the scene, and fast”.