The error states that the column "ch.kills" doesnt exist. Regarding to your script the table-reference `ch` is defined by this line:
$sql = "SELECT $col FROM {$server->charMapDatabase}.`char` AS ch ";
As there is no column called `kills` in the table `char` your script wont work. If you want this to access this script your table `pvpladder` you may change the mentioned line to:
$sql = "SELECT $col FROM {$server->charMapDatabase}.`pvpladder` AS ch ";
Also which ranking script you are using to count the kills ingame? If you want you can add me to skype "ashtorias" or Discoard "Honey#9738"
(no warranty that this will work as I just backtraced your provided informations and official sql-structure of rathena)