What news does Alvin Dewey interject in the courtroom which startles Dick?

Dewey told the court that Perry Smith had confessed to killing all members of the Clutter family. In his confession, he admitted that he, not Dick, had killed Mrs. Clutter and Nancy.

The prosecution's most damaging witness proved to be Alvin Dewey; his testimony, the first public rendering of the events detailed in Perry Smith's confession, earned large headlines (UNVEIL MUTE MURDER HORROR - COLD, CHILLING FACTS TOLD), and shocked his listeners - none more so than Richard Hickock, who came to a startled and chagrined attention when, in the course of Dewey's commentary, the agent said, "There is one incident Smith related to me that I haven't as yet mentioned. And that was that after the Clutter family was tied up, Hickock said to him how well built he thought Nancy Clutter was, and that he was going to rape her. Smith said he told Hickock there wasn't going to be anything like that go on. Smith told me he had no respect for anyone who couldn't control their sexual desires, and that he would have fought Hickock before allowing him to rape the Clutter girl. "Heretofore, Hickock had not known that his partner had informed police of the proposed assault; nor was he aware that, in a friendlier spirit, Perry had altered his original story to claim that he alone had shot the four victims - a fact revealed by Dewey as he neared the end of his testimony: "Perry Smith told me he wished to change two things in the statement he had given us. He said everything else in that statement was true and correct. Except these two things. And that was that he wanted to say he killed Mrs. Clutter and Nancy Clutter - not Hickock. He told me that Hickock... didn't want to die with his mother thinking he had killed any members of the Clutter family. And he said the Hickocks were good people. So why not have it that way."