Cheever is astonished because he never thought the Proctors were capable of such diabolical evil. He'd always trusted them implicitly. But now that he's been presented with what he sees as hard evidence of their involvement in witchcraft, he's absolutely petrified. He knows what happened to Abigail Williams at dinner that night: how she doubled up in excruciating pain, screaming that she'd been stabbed in the belly. Now that Cheever's seen the poppet with the long needle sticking through it, he immediately makes the connection and is now convinced that Elizabeth Proctor is a witch.
Naturally, Cheever's worried that what happened to Abby could just as easily happen to him. Like everyone else in town, he's utterly convinced that there are dark, malevolent forces at work, which could strike him down with death or illness at any moment.