-remove Vader saying “I must obey my master”. Now he just says “You don’t know the power of the dark side”. Vader should be confident in what he believes is right like in ESB, rather than doing it just cause of the Emperor. For goodness sakes, he was plotting to take over before this film.
-remove Vader saying “It is too late for me son.” Way too obvious of foreshadowing, cheapens his eventual turn, and makes Vader seem like he already knows that what he believes in is wrong and misguided. This moment of self revelation for our characters is meant to happen at the END of the arc.
-remove the Emperor saying “You must know by now your father can never be turned from the dark side.” This just sounds plain stupid and makes the Emperor think of Vader as some prize that he has fooled. Vader should feel like a respected ally and learner of the Emperor.
-remove Vader saying “It is useless to resist.” What is this, some sort of shiny object called the Dark side being dangled in front of Luke. This makes the dark side feel less complex.
1 & 2. Darth Vader doesn’t believe he’s doing the right thing. He’s outright sadistic, and power is all that matters to him. That’s the meaning of lines like, “If you only knew the power of the dark side!” and “I am altering the deal. Pray I don’t alter it any further.” He gloats about making Lando his bitch in his face. Twice.
A layer of regressed guilt is ashamed, but he doubles down on being evil because of his ego.
Darth Vader didn’t get high off of lording his power over others and make jokes about killing and torturing people because he thought it was the right thing. He doesn’t care what’s right, it’s about power.
His arc in the film isn’t him finding out the evil magic and killing people is actually bad (which, yeah, he’s a man in his late 40s, he should know that). It’s about choosing love over power. Not knowledge, choice.
3 & 4. I fail to see how the Emperor and Vader presenting the dark side as what it is, a powerful seductive drug, makes it “less complex”. It’s like any other dark magic in any other fantasy story. That’s what it is and always has been.