Compare an evil mastermind in Shakespeare's Othello and MacDonald's Goodnight Desdemona (Good Morning Juliet).

In Othello, Iago is an evil mastermind. Angry that Othello passed him over for a promotion and suspicious that Othello may have seduced his wife, Iago conspires against him. Large chunks of the text are Iago's soliloquies in which he tells the audience how much he hates Othello and how he plans to undo him:

Cassio's a proper man. Let me see now:To get his place and to plume up my willIn double knavery—How, how? Let's see.After some time, to abuse Othello's earThat he is too familiar with his wife.He hath a person and a smooth disposeTo be suspected, framed to make women false.The Moor is of a free and open nature,That thinks men honest that but seem to be so,And will as tenderly be led by th' noseAs asses are.I have 't. It is engendered. Hell and nightMust bring this monstrous birth to the world's light.

In Goodnight Desdemona (Good Morning Juliet), Constance is transported into the story of Othello. Constance meets Othello, Desdemona, and even Iago. Therefore, Iago is still the evil mastermind at play. Due to Constance's interference, Iago now focuses his plans on her:

My wife found this by chance.It's in no Christian hand, but pigeon pecks—Whate'er it be, I will endow it ill,for it must dovetail with my plot. Let's see:To 'venge myself upon the bookish mouse,regain my former credit with my lord,and undo Desdemona once again...How? How? Ah-ha! Confide myselfbetrayed by pendant's lies, to Desdemona!;for she is of a free and noble naturethat thinks men honest that but seem to be.I'll tell her Constance is a spy and whorewould skewer state and marriage on the same cabob!Thus I blind and train my falcon for the job.

In Othello, Iago turns Othello against his wife. In Goodnight Desdemona (Good Morning Juliet), Iago turns Desdemona against Constance. Desdemona becomes a part of his plan, the same way he recruits Roderigo in the original text.
In Othello, Iago's wife, Emilia, gives him Desdemona's dropped handkerchief, which Othello sees as proof of Desdemona's treachery. In Goodnight Desdemona (Good Morning Juliet), Iago tells us that Emilia has handed over one of Constance's manuscript pages.
Iago is an evil mastermind in both plays because of his secret plots and conspiracy with others to undo our heroes.