I mean... the book made it very clear what the "on paper" reason was. Spice production under Beast Rabban had fallen off quite a bit, the Harkonnen's were being replaced by the Atriedes to be more competent and humane. That was the official reason given. "We have to" was because the Duke was given an order by the Emperor, and because no one could refuse that position, even if they didn't want it (as stated above). And the trap was launched and if you recall, the Baron said that after Beast Rabban, the people of Arrakis would welcome their new ruler (Feyd-Rautha). All of this is fairly explicitly stated in the book.

If anything this "bring peace" is silly and takes out much of the nuance that makes any proper movie adaptation so difficult to get right.

J.Ja