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.