Ahem, so if you look at the world that the Jedi and Sith inhabit, it's in a constant state of flux, and whenever one has an overwhelming amount of power, it is easier for one or the other to take over. Even though the character of Palpatine had a great deal of power, it was arguably simpler for him to orchestrate the downfall of the Jedi; they might have felt a disturbance but were ultimately powerless to stop the destruction of their millennia-long rule. Then a mere twenty or so years later, the Emperor was felled by his own minion. It's a very destructive cycle, and one that will never end as long as the two sides oppose one another.
And this is why I have to go with the gray Jedi.
But it's much more than that. There has to be a certain degree of agreement to allow Dark and Light to exist without placing values on them. Both Dark and Light, Chaos and Order, Destruction and Creation have their purposes in this universe, and they exist at the same time. Take reproduction: in order to make a baby, the sperm and egg are destroyed. Do we view the baby as the antagonist? I don't, although I can't make that assertion for everyone. Or when we eat, the nutrients are broken down to create energy, but we don't call our stomach acid evil. We accept both of these instances as natural occurrences, so why not look at Dark and Light through their natural roles. We just have to make sure that one doesn't overwhelm the other.
|Via Star Wars Wikia|
The path of the Gray Jedi isn't necessarily the easiest, even if they do have both sides to draw from. More often than not, you would be considered suspect, ridiculed, or disallowed from higher levels of responsibility, but you do what you want, which makes it totally worth it.
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* In case you can't read the small print: "Grey Jedi Code: There is no Dark Side, nor a Light Side. There is only the Force. I will do what I must to keep the balance. The balance is what keeps me together. There is no good without evil, but evil must not be allowed to flourish. There is passion, yet emotion. Serenity, yet peace. Chaos, yet order. I am the wielder of the flame, the protector of balance. I am the holder of the torch, lighting the way. I am the keeper of the flame, soldier of balance. I am a guardian of balance. I am a Gray Jedi."
** This is how I'm able to keep within the confines of the rules I established for myself, seeing as Gray Jedi weren't really discussed in any of the movies or tie-in novels. But Qui-Gon embodied it so well that it's not technically cheating.