Yeah ... Vader can kill Sidious ... Vader is stronger ... even with his problems as cyborg ... still he could have become Emperor ... I just wonder why he did not killed Palpy on the 20 something years that passed from Epi.III to IV.
"Evil is a word used by the ignorant and the weak. The dark side is about survival. It's about unleashing your inner power. It glorifies the strength of the individual."