What emotion do the words "enemies," "informers," and "spies" appeal to?

Those words appeal to a widespread sense of paranoia in American public life at that time. When Adams wrote his Defense the United States was still a new country, which only recently had prevailed in the Revolutionary War against the British. It's not surprising, then, that there was still a good deal of insecurity about the status of the United States and its place in the wider world. In such an environment it was inevitable that a fierce debate would ensue as to who did and did not belong in this newborn nation.
Then as now, American politics was deeply polarized. It was common for politicians on both sides of the aisle to hurl outrageous accusations at each other, accusing their opponents of being traitors and wanting to sell out the nation, either to the British or the French. Indeed, when Adams subsequently became president, his supporters made precisely such a claim against Thomas Jefferson, claiming that he was in league with the French, working against American maritime interests. And during Adams's presidency, it's notable that he passed the notoriously draconian Alien and Sedition Acts, which were based on the kind of paranoid worldview expressed in words such as "enemies," "informers," and "spies."
http://hua.umf.maine.edu/Reading_Revolutions/Adams.html