Wiles can be defined as devious, cunning stratagems employed in manipulating or persuading someone to do what you want them to do. In "A Poison Tree" by William Blake, it's the speaker of the poem who's deploying his wiles in attempting to persuade his bitter enemy that all is well between them, when the reality couldn't be more different.

The speaker is clearly set on revenge against his enemy. But remembering the old adage that revenge is a dish best served cold, he bides his time, lulling his foe into a false sense of security before it's payback time. At night, when he's alone, the speaker is full of rage and tears, but during the daytime, in front of his enemy, he smiles at him, giving the impression that all's well between them. Yet all along the speaker's hatred toward his enemy continues to grow until, at long last, he exacts his revenge.

The likelihood is that the speaker's enemy never knew what was coming. And this is because the speaker was so successful in employing such devious wiles in persuading him that everything between them was just fine. Although the speaker has succeeded in his task, his behavior is far from admirable, to say the least.