Solved event

https://hub.spigotmc.org/javadocs/spigot/org/bukkit/event/entity/PlayerDeathEvent.html

Code (Java):

@EventHandler
public void playerDeath(PlayerDeathEvent e) {
  // Someone died. Who?
  Player player = e.getEntity();
  // Who killed him? If any.
  if ( player.getKiller() != null ) {
    Player killer = player.getKiller();
    // Do some stuff, like a message, give killer money, etc
  }
}