Anikans decent is evident from the Tuscan raiders in EpII, and the conversion that followed with padme later. “I will be the most powerful Jedi ever!” The hatred starts here. Palpatines influence manifested to the point of murder for revenge. "Revenge is a dish best served cold." His fall to Palpatine starts here. I foresee in EpIII Ani giving up fighting the demons within him. Ob1 telling him he still sees the good him. This leading to the duel that will send Ani from human Ani to "more machine now than man twisted and evil." And Palpatines mind tricks of being "held back" by Ob1 is what leads Ani to fight Ob1. Ani is strong physically in the force but when it comes to his mental strength he is very weak and ruled by his emotions and other outside forces of pursuassion. I think Ob1 realized this and tried to be an outside force but let’s face it Palpatine was much stronger. As far as an oath. Palpatine didn't need one. Vader was easily controlled and influenced. Vader lacked will power until EpVI that is when he showed his true power and brought the balance he was foreseen to bring.