[RandallFlagg]

Never does the series explain why the Siths are evil.

Actually, it does, albeit not in a manner that Lucas might have invisioned.
Imagine what happened after the Rebel Alliance blew up the second Death Star.
The galaxy cheered like crazy, shown in the re-doing of Return of the Jedi.
What they DIDN'T show was the cheering from all of the galactic overlords and tyrants happily patting themselves on the back and arming their henchmen as they said, "WOO-HOO! No more EMPIRE! Now we get to be cruel and dictatorial again to the peoples in our systems!! YAYS!" as they plotted the violent takeover of other worlds.
Jabba times a billion.

[bobjam]

In Episodes 1 and 2, slavery exists on Tattoine. In Episodes 4 and 6 it does not. Go figure.