In most mainstream DC Comics continuity, Ra's al Ghul has been killed and resurrected multiple times. Here are some of the notable instances:
* Batman (various): Batman himself has come close to killing Ra's on multiple occasions, but has ultimately refrained. He's often the one who leaves Ra's to die, but his Lazarus Pit resurrections complicate matters.
* Batman (No Man's Land): In this event, Batman was forced to confront Ra's after he poisoned Gotham City. Ra's was eventually defeated and seemingly killed by the Poison Ivy toxin.
* Batman (Knightfall): Azrael, Batman's temporary replacement, killed Ra's in this storyline, but Ra's again returned from the Lazarus Pit.
* Various villains: Several villains have attempted to kill Ra's, including Talia al Ghul (his daughter), Talia's son Damian Wayne (aka Robin), and other members of the League of Assassins. However, none of these attempts were successful.
In some interpretations, Ra's al Ghul's death is left ambiguous or open to interpretation:
* The Dark Knight Rises (movie): In this film, Ra's is seemingly killed by Batman after falling down a pit. However, his body is never shown, leaving some room for speculation.
* Batman: Arkham City (game): In this game, the player must decide whether or not to let Ra's al Ghul die from his injuries or allow him to escape. The choice ultimately doesn't seem to affect the overall story.
Ultimately, the question of who kills Ra's al Ghul is not so much about a single character as it is about the ongoing struggle between good and evil, and the complex nature of Ra's al Ghul's immortality. His death is often a temporary setback, and his story is likely to continue in one form or another.