He threatens to blow Sirius' head off with his wand, but then decides he'll turn him in to the Dementors…which is a fate worse than death.
Harry uses expelliarmus on Snape, knocking him into a bed. Goodnight, Snape.
Sirius reveals that Scabbers the rat is Peter Pettigrew, who has shapeshifted into a rat.
After prying the rat from Ron's grip, Sirius casts a spell, changing Pettigrew back into a man. A very ratty looking man. And we're grateful that his clothes transform with him.
Pettigrew is begging them for mercy.
Harry says they should spare Pettigrew's life.
He decides they should take him to the castle where the Dementors can have him. That might be a fate worse than being eaten by a cat.