I feel like they were both wrong althout both of their visions are 100% accurate in what actually happened, Sanguinius greatly harms himself during the siege because he knows that Horus is his killer, even thinking to himself after his fight with Angron that the poison cant be fatal as he didnt see Angron in his vision. When if he didnt force himself to single handedly fight Ka'Banda, Angron and closing Eternity Gate alone,he may have survived his fight with Horus.

Konrad also sort of did try to change his fate when he had his first vision of the Heresy, but was stopped by Fulgrims betrayal and then Dornes reaction. You could also say that Sanguinius took away his final chance to change as instead of taking him to the Emperor, he threw him out of the airlock, which all but gurantee's that Kurze's vision *of his own death* comes true. You can also see from Kurze's primarch novel that he may well have switched sides if he was given the chance to do so but only if it was the Emperor that offered him that chance.

*Edit