There are two things that remain for Prospero to do before giving Ariel his freedom? What is the first? What is the second?Are both consistent with Prospero's policy of kindness towards everyone?
Ariel's final two tasks before he receives his freedom are to wakethe master and the boatswain from their slumber beneath the hatches of the ship and bring them before Propsero. Then, Ariel is directed to bring Caliban, Stephano, and Trinculo before Prospero so that they too may be set free. Finally, when this is completed, Prospero says:
In Act V, Prospero stays true to his claim that "the rarer action is / In virtue than in vengeance."
The part you are talking about happens in Act V, Scene 1. There is a long time between Prospero telling Ariel the first thing he has to do and telling him the second.
The first thing that Ariel has to do is to go and release everyone from the spells that they are under.
After that, right at the end, Prospero tells Ariel to arrange to make sure that everyone gets back home safely. Ariel is supposed to ensure that the winds will be favorable.
It seems that these are both pretty nice things for Prospero to do. He does not take any revenge on anyone.