First some context: The entire monologue, beginning with “And what's he then that says I play the villain?” and concluding with “… That shall enmesh them all,” is said by Iago, Othello’s villain. He is describing how he will destroy Othello by using Othello’s wife Desdemona against him. The first part of the speech argues that he is helping the disgraced Cassio by advising him to ask for Desdemona’s help. The second part reveals that this is part of a plot to make Othello jealous of Cassio.

Iago says, “When devils will the blackest sins put on, / They do suggest at first with heavenly shows.” Here Iago is comparing his actions, which are seemingly benevolent, with the wicked suggestions of devils, who are clearly evil. These offenses are not merely bad or sinful; they are “the blackest sins,” the very worst crimes. Iago contrasts this with the “heavenly shows” these devils put on when they suggest evil to unwitting victims. Both Iago and these devils mask their evil intentions with an appearance of goodness.

Later on, Iago says he will turn Desdemona’s “virtue into pitch,” once again contrasting something good (virtue) with something bad (pitch, a dark and sticky substance). These contrasts between good and evil, light and dark, and heavenly and demonic are key to understanding the soliloquy.