I think the jist of the precautionary tale, is that MANUFACTURED messiahs, established only to protect power & grant greater power, are not to be trusted.Yeah, one of the really interesting things that Frank Herbert does with his first 4 Dune books is to set up these contrasting messages about messiah-figures and the concept of "godhead."
On the one hand, he presents them as things to be distrusted, and highlights all manner of awful things that people do when others give them power and ask them to "Fix it for us." The antidote for this is, in a sense, human self-actualization and ingenuity. (I would also argue that you could fit collective efforts into this framework, instead of just saying "everyone always has to go it alone.") The illustrations for this are (1) Paul and the jihad that he unleashes (it's referred to as such in the books, by the way -- that's not me imposing value judgments on it), and (2) Leto II and his oppressive reign. These characters are shown unleashing incredible horror and pain and suffering on the universe and, well, that's bad.
On the other hand, at the same time that he is essentially denouncing godhead and messianic figures, Herbert...arguably proves both Paul and Leto II as having been correct, and as doing what they do because the alternative is human extinction, and both witness this in their visions. Ultimately, Paul doesn't have the capacity to go quite far enough, but he sets the stage for Leto to take on the task himself.
The Children of Dune miniseries plays with this some, showing Leto as a kind of heroic figure making a grand sacrifice...but eliding the fact that the sacrifice he makes is that he'll turn himself into both a literal and figurative monster and will brutally repress humanity for something like 3000 years, essentially to "toughen it up" so that it will be ready for what follows his death and the human diaspora, and then what comes after that.
So, on the one hand, beware of messiahs! They're bad! They do bad stuff and other people do worse stuff in their name! On the other hand, these two messianic figures did awful, awful things...and may have saved humanity in the process. The introduction of prescience into the equation also screws around with this. Assuming your knowledge of the future is accurate (and the books never call this into question, nor do any of the other adaptations), you can know your actions are justified. But Herbert doesn't take the additional step of raising the specter of madness -- that maybe Paul and Leto are just friggin' nutso and their visions are just their own paranoid delusions.
Dune Part 2 sort of teases elements of this with Jessica's transformation after taking the Water of Life, and how crazy she seems. Plus the idea that Alia, a fetus, can communicate telepathically with both Jessica and Paul. Some of this can be played off as saying "Well, that's what happens when you 're pregnant and take the Water of Life." But it could also be used as a way to inject real doubt into the whole notion of the "Golden Path" that features in the next three books.
I believe Paul, & ultimately his progeny made choices that may have aligned with the Bene Geseret's INITIAL plans, at the beginning, but ultimately took their manufactured Messiah completely out of their control.
Ultimately, you could say it was an example of individual love overcoming the 'System', because if Jessica had given birth to a daughter, she would've been wed to Feyd, but instead, she loved Leto so much, she disobeyed the Bene Geseret & gave him the son he wanted. It was simply ironic that generations of planning & scheming by the Bene Geseret, & the backstabbing of the Emperor & Harkonnen, all to solidify their power over humanity, was ultimately the cause of their eventual demise, all because of love.