the Sith say that their purpose it the self-improvement if the individual, and ultimate freedom.
Didn't the Stalinist Communist leadership say the same thing? The use of that word 'freedom' can be quite treacherous.
Ultimate freedom does not come from an individual achieving great power. That is hardly 'self-improvement.' Like alcoholism or drug addiction, it is a way of becoming a slave to your drive, a drive which after a time becomes a compulsive obsession. Striving for power can only lead to a need for more power. I think Schopenhauer understood this; Nietzsche did not.
In the final analysis it is only partly true that power corrupts. If you look deeper, it seems to be true only because only people who are easily corruptible ever seek great power. The man running for high office has, to some extent, already been corrupted.
Ultimate power -- and ultimate freedom -- comes from not seeking, and certainly not needing, anything which, in the end, might corrupt one's inner peace.