But the dark side is not more powerful so it is kinda pointless...
I'd say that the Dark Side is much more powerful, every time a Dark Master have fallen it's because a major Lore Character such as Luke/Anakin/Obi-Wan etc have fought them and won. Anakin lost to Kenobi mainly because his hatred blinded him so much that he only saw red when he should have been strategic in battle and not underestimate Kenobi.
Speaking of Anakin killing of Dooku, Anakin had already begun to turn thus I'd argue that a Fallen Jedi /Dark Jedi defeated a Sith not a Jedi defeated a Sith.
How many Jedi did Grievous, Exar Kun, Jerec or Maul kill before they were defeated?
Yoda vs the Emperor; who won? The Emperor won and Yoda went into exile abandoning and leaving the Jedi to get hunted and slaughtered in masses by the Empire. Thank's Yoda! So much for the light side being stronger.
And I feel that is the answer to your question; in the Star Wars lore the evil force is defeated when it's faced by a challenger that can defeat and overcome the evil. When the evil corrupts the challenger, the challenger either falls(dies) or becomes the new evil. It's because it mainly is a children/teenie story and the formula is easy to understand, follow and it just works!
Aladdin, Beauty and the Beast., Mulan, Jackie Chan Movies, Rocky Movies.. you name it... they all have the same storyline structure, the same difficulties that the hero have to overcome; the hero have to face the badness get beaten by it, rise up and defeat it *add in a 80s training montage here* and it's a great simple formula so why change it?
Reason my character is a Sith? I like the aesthetics more of the Sith and I also love the politics that's going on in the Dark Council and the Empire overall. Agent story is my favourite storyline in this game just for that reason.