Act 4 scene 1 lines 188-192. Prospero to Ariel
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I think that Prospero is referring to Caliban as "a devil". Propspero claims he has tried to help the monster but he only grows uglier and more poisonous. Prospero knows Caliban's plan to have the mortals murder him. Certainly the relationship between Prospero and Caliban is at play here. Prospero, like masters of slaves in any era, feels he is helping Caliban. Caliban plotting for his own freedom makes him, in Prospero's eyes, an ungrateful devil.