The creature bounded through the forest feeling remarkably at ease there. He had spent many a happy hour there with his friends as a student in forests just like this one simply to aid his friend's "furry little problem". Of course, in the form of a dog things always seemed less troublesome to the Azkaban escapee.
So far that morning, he had stolen some chips off a salesman, chased some ducks on a pond and encouraged some children to play with him simply for company (although their parents had been most displeased with the whole thing). Later on, he planned to run through the restricted areas of fields and farmland and hopefully get a good run. He was hungry and weak but all the same, he needed to get to Scotland. Hogwarts School of Witchcraft and Wizardry, to be more precise. He needed to dispose of the rat that had betrayed his best friends to the Dark Lord and therefore caused him to be placed in Azkaban for oh so many years.
And the most important thing he wanted to do? He wanted to prove to his godson, Harry Potter, that he was innocent. That he hadn't done anything to hurt his parents and that he would never have done so if his life depended on it, unlike Wormtail, the true traitor.
Peter Pettigrew, Wormtail, rat, whatever he was refered to as, he was a nasty piece of work that Sirius wanted to have euthanised, preferably against his will. He was sure his old friend Moony would have felt the same way about it. He had heard that he was now a Professor at the school. How joyful that would be! Maybe they would be able to dispose of the vermin together for what he did to Harry's parents? That would be grand!
Sirius was spurred on by that thought. He was desperate to see his old friend again. Desperate to see his godson again. Desperate to get his name cleared and his life back on track. Things would never be the same though. How could they be? His name was tarred with the murder. At least if he could get his name cleared as far as Harry Potter and Remus Lupin were concerned, he would be a very happy dog though.
Every stretch his legs made to carry him closer to the castle felt like an eternity. Every small movement ached in his tired body. And in truth, he was a very scared man, running from the very wizards and witches (spies, almost) who wrongly accused him and were willing to sentece him to death for a crime he didn't (and couldn't) commit. Maybe now he'd get the chance to commit the only murder he had any intent to have committed. It was only a shame that the rat had escaped before the dog could kill it.
And when he had been wrongly imprisoned, he was almost certain he had handled it well. He hadnt created a fuss, apart from repeatedly denying being guilty of course, and he was still doing his best to come to terms with freedom again. Freedom to kill was the only freedom he wanted and they had provided him with an ideal time to come up with the best formula for finally killing the rat.
Now, he just had to implement it. And implement it, he would.