When Leon suposably killed him, he walked away, but if you play Assignment Ada you fight him again. And if you play Seperate Ways, you fight him, and supposedly kill him. Ada watches as his infected arm shrivels up and says " My, that's a big thing you have there. But I don't like it when men play rough."
he's not dead what you saw was his arm regenerating and by the time Ada thought he was dead he was pretty much alive and with his super speed he escaped the island as it was in the process of detonating as you can see see his epilouge from the mercenaries watching the island blow up from the distance but we can't speculate if he's dead or not because only capcom know because they didn't confirm his death or survival for now his status is simply unknown.
I hope not, because he was a pretty cool villain and, at least in my opinion, more badass than Wesker, if not as much of a diabolical mastermind.