Betray [verb] to reveal or disclose in violation of confidence: to betray a secret.

And this was exactly what Peter had done. It was a slip of the tongue, if you wanted to push the boundaries farther than they deserved. His breath had been hot against his cupped palm, pressed against his Lord's ear, shielding it from his enraptured audience. The words had turned to condensation in the creases of his palm, sticky and glistening in the candlelight.

Rat [noun] a person who abandons or betrays his or her party, especially in a time of trouble: he's the rat.

In physical appearance, he resembled a rat: scrawny, with a pointed face and a squeaky voice. His Patronus finally made sense. He had told (he was a tattler he was a snitch he was a talker he was a gossip he was a scoundrel and he didn't deserve anything) on the Potters. He lied about his friendship; he lied to Remus, Sirius and James. He was truly a rat.

Lie [noun] a false statement made with deliberate intent to deceive; an intentional untruth; a falsehood: he had lied.