Who said "Give me a reason to do it, and I swear I will.", and why?

Question

> "Severus, you're making a mistake," said Lupin urgently. "You haven't heard everything -- I can explain -- Sirius is not here to kill Harry --"> > "Two more for Azkaban tonight," said Snape, his eyes now gleaming> fanatically. "I shall be interested to see how Dumbledore takes> this.... He was quite convinced you were harmless, you know, Lupin...> a tame werewolf --"> > "You fool," said Lupin softly. "Is a schoolboy grudge worth putting an> innocent man back inside Azkaban?"> > BANG! Thin, snakelike cords burst from the end of Snape's wand and> twisted themselves around Lupin's mouth, wrists, and ankles; he> overbalanced and fell to the floor, unable to move. With a roar of> rage, Black started toward Snape, but Snape pointed his wand straight> between Black's eyes.> > **"Give me a reason," he whispered. "Give me a reason to do it, and I> swear I will."**> > Black stopped dead. It would have been impossible to say which face> showed more hatred.> > Harry stood there, paralyzed, not knowing what to do or whom to> believe. ...I don't understand, in this context, who actually said "Give me a reason, ..." (Snape or Black?), give a reason to do what and I swear I will do what?

Answer 1

Great question! The threat comes from Professor Snape, who is looking for an opportunity to finally get even with Lupin, Black, and even Harry's dad James.

Picture this: Snape is so angry, and has been for so long, and he finally thinks he is in the right. He won't listen to Lupin, up to the point that he binds and silences him. Black wants to help Lupin, so he heads toward Snape. But Snape gets his wand up first. ("With a roar of rage, Black started toward Snape, but Snape pointed his wand straight between Black's eyes.") Snape is the one in control, and here's the chance for a fight he has thought about for years. That is why he threatens, "Give me a reason." He's threatening and taunting Black, begging Black to give Snape any excuse to put some sort of hex on him--any excuse to get even for all those mean pranks that happened years ago. We know he's the one in control of the situation by the next sentence, "Black stopped dead."

We don't know exactly what he is threatening to do. He is at least threatening to bind him or harm him. Because Sirius Black is a wanted criminal, Professor Snape could do just about anything to him and it would be excused as self-defense.

Answer 2

Since Snape is explained as being the one threatening Black with his wand, because he holds his wand between Black's eyes almost like someone holding someone hostage with a knife or gun, it is Snape that says "Give me a reason...". He has stated this as a challenge or threat to Black that he is fully willing to use the force of his wand against Black if he has some defensible way to explain those actions.