Vecna being disappointed because he wanted to turn Eddie to his side when he turned into a vampire. Eddie's emotions are incredibly heightened, and they're all pointed at Steve. His emotions are stronger than Vecna's mind control.
"Kill him," Vecna said.
They were standing in Steve's room. Eddie was watching Steve sleep.
"Aw, but Henry, he's too cute to kill," Eddie said. "Look at him, I think he's fighting in his sleep."
"Imma kill you," Steve mumbled.
"Aww."
"Kill. Him."
"I shan't," Eddie said. "To slay a sleeping prince with a greater beauty than all of the stars in the universe would be most uncouth of me."
"Oh, for fuck's sake," Vecna swore and Eddie gasped. "Fine, I'll do it."
"Oh, Henry, you really shouldn't have said that," Eddie grinned menacingly.
Steve woke up to the sound of a loud thump. He blinked his eyes several times. Standing there, alive and well, was Eddie Munson. He was holding Vecna's dismembered head in his hands.
"I brought you a present," Eddie said.