Why were Turtle and Sandy dubious about the murder of Samuel Westing?

From the text:

“He looked too peaceful to have been murdered,” Turtle said. She sneezed and Sandy handed her a Westing tissue.

“How would you know?” Doug replied. “How many people have you seen murdered?”

“Turtle’s right,” her friend Sandy said. “If Westing expected it, he’d have seen it coming. His face would have looked scared.”