Knowing the best word that completes a sentence in some cases it might be not easy, however, relying on the contextual clues can give an idea. The term contextual clues refer to the information provided by the words that surround an unknown or missing word. In this case, the contextual clues of the missing word emphasize that the attitude and feeling of the cat is "more than just following his instincts" as the cat "seemed to hate the bird". This shows the missing word should equally express that the feeling goes beyond instincts and that it includes hate. Considering this, the word that completes the sentence is "malevolent" which means malicious or that has a desire to do evil to others, because this word shows the cat hates the bird and that his feeling is more complex than just instinct, which fits with the definition of "malevolent".
The main idea of the passage above from Cotton Mather's "The Wonders of the Invisible World" is to justify the outcome of the trials. Thank you for posting your question. I hope this answer helped you. Let me know if you need more help.
