Wanting to somehow turn back time and do things over (without the meddlesome foolish boyfriend), Jareth somehow managed to hold in the frustrated growl. Sarah wasn't unworthy, as Jareth would never waste his time on someone unworthy. She was, in fact, the only worthy human he had ever met, and that was saying something. The point of the story had been to show her why he did the things he did, and yes, were he being honest, to make her feel badly for rejecting him. This wasn't going according to plan at all.
"You misunderstood." He said, getting to his feet and following them into the kitchen. "Part of it at the very least. Sarah, you are worthy, you are exceptional, and yes, everything I have done, I have done for you. For us...I'm offering you a kingdom, a life apart from this common human nonsense and stench. I want you to come with me, more than anything." Never did he take his eyes off of her, never did his gaze flicker to Peter. He was frightened of what he might do if he was forced to look at the scarred man for even a moment. Cutting her precious boyfriend into pieces might make him feel better, might make the obnoxious sound of his laughter die away, but it would likely drive Sarah further away, and Jareth had not worked for three weeks to do that. No, Jareth worked very hard to ignore him entirely, but his words, his laughter, sunk in anyhow. They were quickly fueling the flame of Jareth's temper, a flame that threatened to explode at any moment into a meltdown.