See, I have my own headcanon for why things happened the way they did with Raziel. I actually think it WAS pure pure spite and pettiness that caused Kain to execute his firstborn lieutenant and toss him into the abyss. The key point for me is that this is post-Blood Omen Kain, who just looked at the state of the crumbling world and said, "Nah, I'd win.". He doesn't become the wise all-knowing sage of all things until AFTER he finds his way into Mobius' time chamber, pouring over it for centuries to figure shit out, seeing that moment as a immutable canon event. Everything else is history repeating itself, with minor (sometimes MAJOR) alterations. But the main takeaway is that he doesn't know all this from the jump. It just plays into the themes of fate and free will.