Why did Remus and Sirius fight in Prisoner of Azkaban (werewolf vs. dog)?

Question

In Prisoner of Azkaban, Remus Lupin tells Harry, Ron, and Hermione that werewolves are dangerous only to humans:

‘They couldn’t keep me company as humans, so they kept me company as animals,’ said Lupin. ‘A werewolf is only a danger to people.

_{Prisoner of Azkaban - page 260 - Bloomsbury - chapter 19, Moony, Wormtail, Padfoot, and Prongs}

Yet, after Remus transforms following the events in the Shrieking Shack that led to Peter Pettigrew's identification, Remus's admission that he was a werewolf, and Sirius Black's status as a dog Animagus, while on the way back to the castle, Remus transforms into a werewolf and he and Sirius (in Animagus form) get into a pretty vicious physical fight.

As the werewolf reared, snapping its long jaws, Sirius disappeared from Harry’s side. He had transformed. The enormous, bear-like dog bounded forwards. As the werewolf wrenched itself free of the manacle binding it, the dog seized it about the neck and pulled it backwards, away from Ron and Pettigrew. They were locked, jaw to jaw, claws ripping at each other –

_{Prisoner of Azkaban - page 279 - Bloomsbury - chapter 20, The Dementor's Kiss}

But then, out of the darkness, they heard a yelping, a whining; a dog in pain ...

‘Sirius,’ Harry muttered, staring into the darkness. He had a moment’s indecision, but there was nothing they could do for Ron at the moment, and by the sound of it, Black was in trouble –

_{Prisoner of Azkaban - page 280 - Bloomsbury - chapter 20, The Dementor's Kiss}

I realize Sirius went after Remus first, so Remus probably reacted instinctively and fought back initially. But why would he continue to fight physically after gaining the upper hand, to the point where Sirius was seriously injured? Remus had made the point that when his friends, transfigured as Animagi, were with him while he was a werewolf, it helped to calm Remus and help him tolerate his werewolf transformation.

Why on this particular occasion did Remus attack Sirius so viciously when Sirius was in his dog form? Shouldn't Sirius's presence as a dog have helped to calm Remus rather than incite him?

Answer 1

I remember there being a lot more that's relevant to the question in that chapter of the book, but unfortunately don't have access to it right now. I'll do my best to paraphrase from memory, and update with quotes when I'm able (not until much later today at the earliest).

I recall Lupin saying that James and Sirius, transforming into a stag and dog respectively, were large enough to keep him in line, and that the adventures they had in the Hogwarts grounds during his transformations made the rest of the time tolerable.

I think there's a large difference in the mindset Lupin would be in during the relevant transformations. During his time at Hogwarts he would have been relatively happy, and prepared to transform knowing that while he was dangerous, there would be James and Sirius there to prevent him from doing any harm.

On the other hand, at the end of Prisoner of Azkaban Lupin would be used to transforming under the effects of the Wolfsbane Potion and likely wouldn't be approaching the transformations in quite the same way he did at school. You also have to consider that it was an emotionally charged time for all involved; Lupin had just discovered that Peter Pettigrew was still alive and that it was Peter rather than Sirius that betrayed Harry's parents to Voldemort.

It was likely a combination of human emotion and animal instinct causing what could be described as bloodlust; Sirius was the first target of opportunity (likely by design) and he fixated on him while ignoring everybody else.

Answer 2

I don't think there's a canon answer, but if he was in a true canine mode, Sirius was between him and his prey. Wolves get somewhat annoyed when you do that, and I remember reading stories about shepherd dogs hurt by wolves trying to hunt "their" cattle/sheep. And werewolf would be even worse than a real wolf since he wouldn't have a wolve's main incentive to NOT fight for the access to prey (namely, getting injured is basically a death sentence since you can't hunt effectively anymore).

Answer 3

As is noted in the comment to the question, Sirius had to attack Lupin to protect the kids and the other two men. Remus fought viciously this time because of the reasons Anthony Grist and DVK provide. And Sirius was "bleeding with gashes across his muzzle and back" too.

But beyond that, the premises in the question seem to be wrong/confusing.

For, Sirius was in fact able to drive Lupin away.

"There was a howl and a rumbling growl; Harry turned to see the werewolf taking flight; it was galloping into the forest. "

The second part of the second quotation in the OP occurs after this when Harry informs the bleeding Sirius of Pettigrew's escape, at which he "scrambles up again" and pursues Pettigrew.

It was still after a while, thinking of moving Ron and Snape to the castle, that Harry and Hermione hear "out of the darkness", "a yelping, a whining; a dog in pain".

And they ran in the direction to discover the reason: Sirius had turned back into a man, and was crouching, all because the dementors were swarming.

Which brings up another question: Why did Sirius succumb so easily to the dementors that he was even forced out of his Animagus form?

Answer 4

The transformed Remus wanted to attack the humans that were there, especially Ron. Sirius was dragging him away from them, and Remus wanted to get there, so he had to fight.

Answer 5

Sirius was trying to protect his godson, and (Harry's) friends.

Keep in mind that Remus was not sentient, when in werewolf form, so using reason was not an option.

Sirius didn't really have a choice here; Wormtail had already betrayed the Potters before, there was no way Sirius would back down.

Answer 6

I think the reason why Remus or Remus as a werewolf was so aggressive was because Ron was bleeding? Not sure though. And he must have had some sort of bloodlust since he was always in the Shrieking Shack during PoA transformations and never really had a prey prior to this.

Sirius being Sirius, he tried to protect Remus, first of all, from himself, because he would never forgive himself if he killed Ron, or Harry or anyone because he's Remus. And SB was also saving Harry and making sure that he was safe. And the reason why their fight was so vicious was probably because Padfoot came between Moony and his preys.