From "Manual of Muad'Dib" by the Princess Irulan

Soon I must remove him , the Baron thought. He has almost outlasted his usefulness, almost reached the point of positive danger to my person. First, though, he must make the people of Arrakis hate him. Then—they will welcome my darling Feyd-Rautha as a savior .
The Baron shifted his attention to the guard captain—Umman Kudu: scissors-line of jaw muscles, chin like a boot toe—a man to be trusted because the captain's vices were known.
"First, where is the traitor who gave me the Duke?" the Baron asked. "I must give the traitor his reward."
Piter turned on one toe, motioned to the guard outside.
A bit of black movement there and Yueh walked through. His motions were stiff and stringy. The mustache drooped beside his purple lips. Only the old eyes seemed alive. Yueh came to a stop three paces into the room, obeying a motion from Piter, and stood there staring across the open space at the Baron.
"Ah-h-h, Dr. Yueh."
"M'Lord Harkonnen."
"You've given us the Duke, I hear."
"My half of the bargain, m'Lord."
The Baron looked at Piter.
Piter nodded.
The Baron looked back at Yueh. "The letter of the bargain, eh? And I . . ." He spat the words out: "What was I to do in return?"
"You remember quite well, m'Lord Harkonnen."
And Yueh allowed himself to think now, hearing the loud silence of clocks in his mind. He had seen the subtle betrayals in the Baron's manner. Wanna was indeed dead—gone far beyond their reach. Otherwise, there'd still be a hold on the weak doctor. The Baron's manner showed there was no hold; it was ended.
"Do I?" the Baron asked.
"You promised to deliver my Wanna from her agony."
The Baron nodded. "Oh, yes. Now, I remember. So I did. That was my promise. That was how we bent the Imperial Conditioning. You couldn't endure seeing your Bene Gesserit witch grovel in Piter's pain amplifiers. Well, the Baron Vladimir Harkonnen always keeps his promises. I told you I'd free her from the agony and permit you to join her. So be it." He waved a hand at Piter.
Piter's blue eyes took a glazed look. His movement was catlike in its sudden fluidity. The knife in his hand glistened like a claw as it flashed into Yueh's back.
The old man stiffened, never taking his attention from the Baron.
"So join her!" the Baron spat.
Yueh stood, swaying. His lips moved with careful precision, and his voice came in oddly measured cadence: "You . . . think . . . you . . . de . . . feated . . . me. You . . . think . . . I . . . did . . . not . . . know . . . what . . . I . . . bought . . . for . . . my . . . Wanna."
He toppled. No bending or softening. It was like a tree falling.
"So join her," the Baron repeated. But his words were like a weak echo.
Yueh had filled him with a sense of foreboding. He whipped his attention to Piter, watched the man wipe the blade on a scrap of cloth, watched the creamy look of satisfaction in the blue eyes.
So that's how he kills by his own hand , the Baron thought. It's well to know .
"He did give us the Duke?" the Baron asked.
"Of a certainty, my Lord," Piter said.
"Then get him in here!"
Piter glanced at the guard captain, who whirled to obey.
The Baron looked down at Yueh. From the way the man had fallen, you could suspect oak in him instead of bones.
"I never could bring myself to trust a traitor," the Baron said. "Not even a traitor I created."
He glanced at the night-shrouded viewport. That black bag of stillness out there was his, the Baron knew. There was no more crump of artillery against the Shield Wall caves; the burrow traps were sealed off. Quite suddenly, the Baron's mind could conceive of nothing more beautiful than that utter emptiness of black. Unless it were white on the black. Plated white on the black. Porcelain white.
But there was still the feeling of doubt.
What had the old fool of a doctor meant? Of course, he'd probably known what would happen to him in the end. But that bit about thinking he'd been defeated: "You think you defeated me ."
What had he meant?
The Duke Leto Atreides came through the door. His arms were bound in chains, the eagle face streaked with dirt. His uniform was torn where someone had ripped off his insignia. There were tatters at his waist where the shield belt had been removed without first freeing the uniform ties. The Duke's eyes held a glazed, insane look.
"Wel-l-l-l," the Baron said. He hesitated, drawing in a deep breath. He knew he had spoken too loudly. This moment, long envisioned, had lost some of its savor.