What's so bad about the dark side anyway?
Ahem, to answer the question, nothing. Its a perception, a point of view, like Palpatine said to Anakin. The Dark Side is a perception, both politically and traditionally. The Dark Side is the manifestation of one's emotion formed through rage, suffering, loss, or ambition. There is nothing bad about it, its just others' perception of the Dark Side and its view of the force, both natural and artificial. Once can interpret the Dark Side in many ways, but there is no definite Evil about it. Even if you are a converted Dark Jedi or a Sith, the notion of religious good and evil still exists. This goes the same with the followers of the light side. There is evil, deceit, corruption, disregard, and hate. There is non-violence, diplomacy, respect, and forgiveness. What makes these both true ideals, and not just the perception of a side is consequences. The Sith do not take into account retribution, innocence, and the the regard of others but yourself. They use others. The Jedi Code will have none of this. Evil and corruption does exist, destroying the billions of lives upon a planet is a morally determined example the Sith do not face / justify. The Jedi do not have their noses involved with such - if they are viewed as weak for valuing life, wars are bound to be fought. But a faction working for the good of a whole, perhaps something like the Republic, cannot be ignored and its prosperity lasted countless years. The Prosperity of the Sith Rule, think of this? What was it - not even half a century? And before, always a broken up power? There certainly is a difference and significance of both good and evil.
To use an allagory:
There is a river that runs through a valley that provides water and nutrients to all the plants and the animals within that valley. There is also a native group of people who use this water for growing food and drinking out of. This river is the force and the natives are the Jedi.
The river is balanced when it is able to find it's natural route while the natives draw from it to sustain their lives.
Now picture a non-native group of people who decide to dam up the river and build up the banks. The feel that they need to control the river to not only survive, but to expand their power (as opposed to the natives who draw from it in its natural course). These non-natives are the sith who seek to control the force as opposed to listen to it and draw from it.
There is no definitive Evil about the Dark Side. The Dark Side is not Bad by any means. It doesn't matter whether there were Humane Sith Lords or not. The world is filled with opinions of Good and Bad. Its how those select few control the Dark Side to their advantage. The creator of Star Wars states that the Dark Side is indeed bad, but Creators aren't always right, even though the universe is created through their mind. Its a matter of perception, what you think is bad and I think is bad can be two different Evils if you will, the only definitive answer to Good and Evil is Religious Tradition, and that perception of Good and Evil is not always clear, or right by any means.
As for justification in Good and Evil, again, no definitive answer. Anyone can have different agendas for such means they'll go to either be Good or Bad. Whether one meets an end, there are always going to be people's perception on whether what one did was good and bad. Majority's perception isn't always right, and the Minority's perception is always shunned by Society for being Different in all means.
The Dark side and Light side aren't inherently 'good' or 'bad'; to imply that they are is to imply that the Force is a self-aware, thinking being. The Force is exactly what the name implies; a neutral force of nature. Arguing whether or not it is good or evil is like arguing over whether or not fire or electricity is good or evil. Both are simply tools that can be used either benignly or malignantly
I also don't see Emotion as a Dark Side trait. Emotion is not a Dark Side trait, it is an inescapable trait in Human Beings, not only Dark Side Users, but Light Side Users as well, and Emotion is always going to be factor, whether a tiny portion or large portion, of your decisions, regardless of whether you're aligned with the Dark Side or not. You can't honestly say that Emotion is a Dark Side trait, because it isn't. It is, however, abused in the Dark Side of the Force, and influenced, but it never really is connected to the Dark Side.
Bottom line, there is no Dark nor Light side of the Force. There is only the Force. Even the Jedi Code states "....there is the Force" not the light side of the Force. The Force is neither good nor evil. It just is. There is no consiencness that the Force has. It guides and controls on how much we let it. Nothing without consiencness can make decisions on doing good or bad things. Only the users make that decision. Luke asked Obi Wan if the Force controls your actions, his response, "Partially, but it also obeys your commands." That right there proves that the Force is dark or light depending on how we use it. The Dark side of the light side is a point of view. When Jedi fight, they are aggressive, which leads to the dark side. But it only leads to the dark side by releasing and giving in to your emotions. I'm saying that the Force is neither good nor evil. How can it be? It just is. You tell me that the Force started in darkness, making it evil and the Jedi made it light as being the beacon of light. That's a metafore. I agree about the Jedi being the beacon of light, but not in a sense that they made the Force good nor evil. It's still a perception. To say that the Force is evil is like saying a tree is evil. A tree is neither. Forget the fact that a tree needs light to survive, but its the actions of the tree. The tree doesn't do evil or good things. If a tree falls on a person and kills them, that's evil. But, the tree didn't fall by itself. Something made it fall, whether it was wind or it was cut down, or whatever. That's what the Force is like. It doesn't do evil deeds unless it is made to do so. That's why it is neither good nor evil. Dark side, light side, it's just a perception of how people use the Force. A Perseption. Period.