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human
[Star Wars] When did Vader become disillusioned with the Emperor?

Was there a single event that made him begin to question his choices?  How close, if at all, did he get to turning on Palpatine before the events of ROTJ?
gpt
Depends. In legends it was within the first few days of the empire that palpatine put vader to work pacifying a separatist world. Palpatine was enslaving the former cis rather than abolishing slavery. While on this assignment vader asked a clone trooper if there was an order similar to 66 that would turn the troops against him. When the soldier admitted he couldn't divulge such an order if it did exist vader killed him to prevent palpatine finding out. After that he spent much of his free time gathering and training assassins to potentially kill palpatine but was always one step behind his master. 

In canon vader seems to be doubting his choices constantly but it isn't until tarkin is elevated above vader that Vader starts to seriously plot against his master. Sometime after the death star's destruction Vader creates his own private army of droids to try and outplay palpatine from the shadows but again is always a few steps behind palpatine. He never fully trusts palpatine but after discovering palptine had been grooming replacements since vaders wounds on mustafar Vader actively plots to kill Palpatine but fumbles in the dark trying to find allies. 

Both continuities vader ultimately fails until the battle of endor and the only major difference is vader is secretly against Palpatine as early as 19 bby in legends whereas in canon it's not until 0ABY.