Ferret of the bearded persuasion,
It's not as so much as Rei winning the Rei/Tremayne fight; it's more of actually winning against the Dark Side in a *very* tight position. Falling into a hole was merely cinematic presentation, as high space operas go--you've read the effect.
To be honest... it really is tempting to give in to the Murderer's Edge when it offers you a hand in the middle of a very vicious fight against someone truly more skilled/talented than you. That's the whole point of the fight--does it make or break her control of the Dark Side?
(BTW, "Murderer's Edge" is also the name of the book series my brother is writing; the idea itself isn't that dissimilar to the Dark Side, except that it's just pure primal fury. Very handy if you're a mercenary stuck in a stupid war on the hind-end of the world, and led by the most arrogant men possible. It's over on Blackwyrm Publishing--www.blackwyrm.com--feel free to check it out, even buy it.)