So if I torture you for no apperant reason I'm just trying to enforce an archaic view of catagorization on you, simply for the reason of making you act a certain way?
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Let's for arguments sake just assume that GL was trying to make a statement about the inherint qualities in humans. Saying that emotions like greed, anger, fear and hatred were all associated with corruption of the mind and soul - while on the opposite side benevolance, compassion, forgivness and respect were signs of enlightment and purity of mind and soul.
But the question posed in this thread is "What's so bad about the dark side anyway" - well it's a symbol of human weakness and that classical statement "the road to Hell is paved with good intensions".
You're essentially asking what's so bad with fear, anger, hate and greed. One of the most simple answers would be, that such emotions causes us to be clouded by our own illusions. On the other hand though - compassion, benevolance and forgivness without true purpose is just as dangerous - but that's why the road to Hell is indeed paved with good intensions.
Edit: Sorry about the double posting - my browser went insane for a moment.