Perhaps a bit obvious to be the answer, but is it Master Tonberry?

As the saying goes
What goes around comes around
To all that I choose to dispose - A reference to the Karma attack, the damage of which is based on how many enemies the targeted character has previously defeated.
But don’t be fooled for I’m not my predecessor - It's predecessor is the Tonberry, which is lot weaker.
Me, I’m more of a rector - This would make a lot of sense if we remove a large portion of the religious connotation from the word "Rector", leaving it just to mean 'Master', 'Leader' or anything of that like.