What did Richard see on the faces of the Duke and his son after the unicorn had been slaughtered in the novel, De Natura Unicorni?

When Richard witnesses the unicorn's grizly death, he suppresses the urge to vomit, and when he turns to look at the duke and James: "Richard was not sure what he expected—what terror and what pain. But he was not prepared for the look of ecstasy on the faces of both father and son."