“That if you now beheld them, your affections would become tender.” (act 5 sc. 1, line 23-24)
From this quote it is evident that this individual is stating, if said person were to see it for themselves, the individual would feel some sorrow or some emotion. If one were to see ‘them’, them being traitors of Prospero (Alonso, Antonio and Sebastian), one would empathize for their state of being assuming is not at its best. This quote comes from this spirit named Ariel, who is not human, thus the quote further demonstrates the spirit’s capability of feeling sorrow or remorse for these individuals that he is wary of.
Ariel has seen these individuals plot to kill one of the Kings, King Alonso shortly after arriving on the island. So his perspective of these people is already altered and can see the corruption among-st the noblemen. These noblemen are very opportunistic and go after individuals when they are at their weakest or vulnerable, as they attempted to kill the king in his sleep. Yet with this perspective still feels for the pain and suffering that they experience due to Prospero’s magic and more. Prosperos vengeance stems so deep that he fails to see the inhumane nature of his spells and ill-doing. However a mythological creature sees the wrong in his actions and speaks up on it, he brings it to his masters attention, only to hopefully bring about justice. In return Prospero asks that Ariel relieve them of their suffering and later on admits to his actions and why he has done so in a grand unveiling.
I agree that for a nonhuman being such as Ariel to develop these feelings of remorse and pity towards these corrupt people, what Prospero was doing must have been something really awful. Ariel have witnessed all the abominable traits in these characters through observing them and reporting back to Prospero and through interaction with them in carrying out Prospero’s commands. Like you said, Ariel has observed Antonio convince Sebastian that he is the rightful heir to the throne and the two plotted to kill Alonso. This goes on to show just how blinded Prospero is in his drive for vengeance and reestablishment of his former glory.