Prospero has long wished to wreak vengeance on Alonso, Antonio, and the others who conspired to depose him from the throne. When Ariel reports that the men are trapped terrified in the grove, unable to move, Prospero knows he has them where he wants them and can punish them as he wishes.

Alonso, the king of Naples, is not a terrible person, though he is weak-willed and was convinced too easily by Antonio to help him in his plot to usurp the throne. Because of what Alonso enabled Antonio to do, which might easily have cost Prospero and Miranda their lives (they were set out to sea in a leaky ship), Prospero is justified in imprisoning him in the grove. On the other hand, Alonso is less to blame for what occurred than Antonio, and Alonso does repent and apologize for his acts, unlike Antonio.

Therefore, while Alonso is not the worst of the lot, he is hardly guilt-free and does deserve what he gets for what he has done. However, the point of the play is that Prospero chooses not justice (he would be justified in taking revenge) but mercy—what he describes as the higher and rarer road. He chooses this path because he witnesses Ariel's distress at the plight of the guilty men. Prospero feels that if a mere spirit can feel forgiveness and mercy, he as a human is all the more compelled to follow the path of compassion—and so he does.

A little bit of terror in the grove does not seem a high price to pay for the plot against Prospero that Alonso was part of. It was hard on Alonso when he believed Ferdinard was dead, but that too has a happy ending.