Ascension: Killed an enemy but he ascended afterwards

I killed the enemy Kha'zix before the ascension objective thing was killed and he somehow stole it and was brought back to life as the ascended despite being dead and not having anything that did damage over time (like a poison or bleed). pretty sure this is a bug because killing the ascension thing imediatly activates a zhonyas-like effect so I would not have been able to kill him if he had taken it.