Perhaps it was the case that Magnus was just faced with utter despair. The realization that an entity more cruel and intelligent than he could possibly imagine was influencing his destiny perhaps even before he was born, as well as the destiny of the entire universe is bound to give a person pause for. When confronted with the truth, of his utter helplessness maybe he realized that it was futile to fight against such a being. Magnus considered himself above his "primordial ancestors" and thought that he could shepard them into a golden age. Now he realized that there is only one true master and that this "Golden Age" would end up empowering it. Why not serve Tzeentch, after all if he chose not to, he'd probably end up serving him anyway, so the best course of action is to embrace him. Such is the way with Tzeentch, whatever you do, he'll be pulling the strings.
"Death? You want death? I'm afraid that would be far too boring."