It's not certain the emperor would have been killed in the death star explosion. Maybe he could have escaped in the last moments like Luke eventually did.

Luke couldn't have handled the Emperor by himself. Not enough power. And if he did somehow kill the emperor he'd have probably gone dark because of doing it in anger. But I think Vader was very destined to be there. And I'm not just talking about the bloody prophecy. Destiny is mentioned in the OOT and I think it's reasonable to think that in a fairytale like Star wars the characters were destined to fulfil certain roles. Vader was destined to be there and the only question was what side he'd end up taking. Also, Luke wasn't just going to go hand himself over to any old imperials just o meet the emperor. He handed himself over because he knew Vader was there. Meeting Vader was a certainty.