To the Opening Poster:
Yoda is as capable of any Jedi at being "tempted." I think that's an easy enough assumption. Anakin, Luke, Count Dooku.... but not Yoda? I mean sure he's old and powerful, but so is Palpatine.
Write your fan fiction and do whatever you want. Honestly that moment never had me pegged as a "Dark Side moment" but who knew what he was thinking? To my mind the scene with him and Dooku in the hangar in AOTC is a stronger candidate, but who knows, write what you want. Lucas isn't tell you what to do in your story... right?
People have written crazier fan fics out there, so if you can do it, I say do it.
The thing is, so many of us have been trained by the "brain bug" of the video games, to think that certain powers are only usable by "Dark Jedi" (or Sith) and others only by "Lightside" Jedi. Luke chokes two Gammorean guards in Jabba's palace. Did he get tempted by the Dark Side in that moment? The movie leaves it wide open and never tells us (though the books might have said a thing or two, I'm fairly sure the novel says he did indeed choke them, not just "make them fall asleep" or something).
Likewise, Yoda uses lightning in AOTC, according to the movie. People refuse to believe this, insisting that lightning is only a darkside power, which they've learned from the video games and RPG's. Sure, the ROTJ novelisation calls it "evil lightning" (iirc) and something Luke thought was a "corrupt use" of the Force. However remember how many "rules" we thought we knew were overturned in the prequels. Look at what a warrior Yoda was, despite his apparent disdain for the use of violence except in self defense in ESB. Look at how fast the old Jedi move in battle, despite Lucas' previous explanation that Ben and Vader had to fight slowly on the Death Star because they were "old Jedi, and one of them was a cripple... so it was a hard fight."
And Yoda himself isn't infallible or all powerful. Remember the mistakes he made? Remember how he misjudged people, and ran away from combat? He looked a bit angry when he killed those troopers on Kashyyyk too, now that I think of it... and who wouldn't be, there had just been a purge of the Jedi generals, and he was next. Yoda thought nobody could come back from the Dark Side, but Anakin did. He thought Luke facing Vader on Bespin would "destroy all for what they had fought and suffered" (the Rebellion), but that didn't happen either.
The EU writers have long looked for ways to find "loopholes" to get "good" Jedi to use force lightning. So they invent stuff like "electrict justice" (or whatever its called) and have people be "tempted" but come back after they've used their power, etc.
If using Force Push in combat against a foe is "Dark Side" then tons of the Jedi are guilty. If using force choke on a living foe is "Dark Side" then Luke is guilty. If attacking somebody first is "Dark Side" then lots of the Jedi are guilty. Etc...
Is this a "what if" story? Again, if so, no problem there at all.
I mean, let's face it, you could write a story in which Obi-Wan Kenobi is a closeted homosexual. As long as it's well written, even if the idea seems silly, why not, right? Unless you start selling it, or Lucas sends you a Cease and Desist Order, all's fair...