Harry made up his mind in a split second. Before Snape could take even one step toward him, he had raised his wand.
To make matters even worse, Harry's anti-Dementor lessons were not going nearly as well as he had hoped. Several sessions on, he was able to produce an indistinct, silvery shadow every time the Boggart-Dementor approached him, but his Patronus was too feeble to drive the Dementor away. All it did was hover, like a semitransparent cloud, draining Harry of energy as he fought to keep it there. Harry felt angry with himself, guilty about his secret desire to hear his parents¡¯ voices again.
Harry raised the wand. Now was the moment to do it. Now was the moment to avenge his mother and father. He was going to kill Black. He had to kill Black. This was his chance¡.