Program: 10414
Project: 10414
Buggy source:
@EventHandler(priority=EventPriority.MONITOR) public void onDeath(EntityDamageEvent event){
  if (event.getEntity() != npc.getBukkitEntity() || dying)   return;
  if (this.getHealth() - event.getDamage() > 0)   return;
  dying=true;
  player=null;
  String deathCause=event.getCause().toString().toLowerCase().replace('_',' ');
  if (event instanceof EntityDamageByEntityEvent) {
    Entity killerEntity=((EntityDamageByEntityEvent)event).getDamager();
    if (killerEntity instanceof Player)     player=(Player)killerEntity;
 else     if (killerEntity instanceof Projectile) {
      LivingEntity shooter=((Projectile)killerEntity).getShooter();
      if (shooter instanceof Player)       player=(Player)shooter;
      DenizenAPI.getDenizenNPC(npc).action("String_Node_Str" + shooter.getType().toString(),player);
    }
    DenizenAPI.getDenizenNPC(npc).action("String_Node_Str",player);
    DenizenAPI.getDenizenNPC(npc).action("String_Node_Str" + killerEntity.getType().toString(),player);
  }
 else   if (event instanceof EntityDamageByBlockEvent) {
    DenizenAPI.getDenizenNPC(npc).action("String_Node_Str",player);
  }
  DenizenAPI.getDenizenNPC(npc).action("String_Node_Str",player);
  DenizenAPI.getDenizenNPC(npc).action("String_Node_Str" + deathCause,player);
  if (npc.getBukkitEntity() == null)   return;
  loc=aH.getLocationFrom(DenizenAPI.getCurrentInstance().tagManager().tag(null,DenizenAPI.getDenizenNPC(npc),respawnLocation,false));
  if (loc == null)   loc=npc.getBukkitEntity().getLocation();
  if (animatedeath) {
    setHealth();
    npc.getBukkitEntity().playEffect(EntityEffect.DEATH);
    Bukkit.getServer().getScheduler().scheduleSyncDelayedTask(DenizenAPI.getCurrentInstance(),new Runnable(){
      public void run(){
        die();
      }
    }
,(long)((Duration.valueOf(animationDelay).getSeconds() * 20)));
  }
 else {
    die();
  }
  if (respawn) {
    Bukkit.getServer().getScheduler().scheduleSyncDelayedTask(DenizenAPI.getCurrentInstance(),new Runnable(){
      public void run(){
        if (npc.isSpawned())         return;
        npc.spawn(loc);
      }
    }
,(long)((Duration.valueOf(respawnDelay).getSeconds() * 20)));
  }
}

Fixed version (ground truth):
@EventHandler(priority=EventPriority.MONITOR) public void onDeath(EntityDamageEvent event){
  if (event.getEntity() != npc.getBukkitEntity() || dying)   return;
  if (this.getHealth() - event.getDamage() > 0)   return;
  dying=true;
  player=null;
  String deathCause=event.getCause().toString().toLowerCase().replace('_',' ');
  if (event instanceof EntityDamageByEntityEvent) {
    Entity killerEntity=((EntityDamageByEntityEvent)event).getDamager();
    if (killerEntity instanceof Player)     player=(Player)killerEntity;
 else     if (killerEntity instanceof Projectile) {
      LivingEntity shooter=((Projectile)killerEntity).getShooter();
      if (shooter instanceof Player)       player=(Player)shooter;
      DenizenAPI.getDenizenNPC(npc).action("String_Node_Str" + shooter.getType().toString(),player);
    }
    DenizenAPI.getDenizenNPC(npc).action("String_Node_Str",player);
    DenizenAPI.getDenizenNPC(npc).action("String_Node_Str" + killerEntity.getType().toString(),player);
  }
 else   if (event instanceof EntityDamageByBlockEvent) {
    DenizenAPI.getDenizenNPC(npc).action("String_Node_Str",player);
  }
  DenizenAPI.getDenizenNPC(npc).action("String_Node_Str",player);
  DenizenAPI.getDenizenNPC(npc).action("String_Node_Str" + deathCause,player);
  if (npc.getBukkitEntity() == null)   return;
  loc=aH.getLocationFrom(TagManager.tag(null,DenizenAPI.getDenizenNPC(npc),respawnLocation,false));
  if (loc == null)   loc=npc.getBukkitEntity().getLocation();
  if (animatedeath) {
    npc.getNavigator().cancelNavigation();
    setHealth();
    npc.getBukkitEntity().playEffect(EntityEffect.DEATH);
    Bukkit.getServer().getScheduler().scheduleSyncDelayedTask(DenizenAPI.getCurrentInstance(),new Runnable(){
      public void run(){
        die();
      }
    }
,60);
  }
 else   die();
  if (respawn) {
    Bukkit.getServer().getScheduler().scheduleSyncDelayedTask(DenizenAPI.getCurrentInstance(),new Runnable(){
      public void run(){
        if (npc.isSpawned())         return;
 else         npc.spawn(loc);
      }
    }
,(Duration.valueOf(respawnDelay).getTicks()));
  }
}