If I'm remembering correctly, in the book it was more clear that his motivation was:
1. To end his wife's suffering - in the event she was still alive, him carrying this out would either mean her death or her release, either way her suffering would be over.
2. Really, the main thing he hoped to accomplish was get in position to kill the fukking baron. He didn't trust the Baron, assumed he would kill both him and his wife if he hadn't already, so the duke would be the bait to at least give him a chance at revenge.
Read again, I think book readers are just as split as non-readers. Several of us big Dune fans liked the movie but would have preferred it was longer and explained more. Like I said, I think about 20 more minutes to set up a few more storylines better would have been doable.
On the other hand, movie already over $300 million at the box office, 82% critics and 90% audience on Rotten Tomatoes, 8.2 rating on IMDB, and the vast majority of all those tickets and positive reviews are the work of non-readers who loved the movie despite the lack of exposition.