There is a single line that tries to explain this..."When you kill another, you kill yourself". The dream was supposed to show that by Paul killing this man, he was also killing the naive boy (Himself), so the Kwisatz Haderach could rise.
I think it was just supposed to show the audience why prescience hasn't been able to produce a god emp so far...Because seeing the future and how your brain interprets it, doesn't always translate into clear images. Its a major plot point of the book, how overwhelming and opaque prescience actually is. (And Paul can overcome it because he is both a Bene Gesserit and a Mentat.
I think the movie was trying to show that difficulty by making the visions 'true' but in odd ways....So the black dude did teach Paul, but not in the conventional way...(Taught him how to literally become a man in the fremen's eyes).
There was also another aspect they seemed to downplay. In the fight, the black dude swaps his knife hand at the end...behind his back. I think the vision was supposed to warn Paul of that, but they didn't do a very good job of conveying that visually.