What was McMurdo’s response to Ettie’s little mischief? Why do you think he responded the way he did?

From the text:

If she had expected to startle him, she certainly succeeded; but only in turn to be startled herself. With a tiger spring he turned on her, and his right hand was feeling for her throat. At the same instant with the other hand he crumpled up the paper that lay before him. For an instant he stood glaring. Then astonishment and joy took the place of the ferocity which had convulsed his features—a ferocity which had sent her shrinking back in horror as from something which had never before intruded into her gentle life. “It’s you!” said he, mopping his brow.

“And to think that you should come to me, heart of my heart, and I should find nothing better to do than to want to strangle you! Come then, darling,” and he held out his arms, “let me make it up to you.”

From the text, we can infer that McMurdo believed he was about to be attacked... he was always on guard.