For having red lightsabers? For wearing black robes? They're allowed to feel passion and attachement? If anything, the Jedi are evil for denying themselves those things. And Darth Plageuis for instance, he wanted to preserve life. What's so evil about that?
[Edited on 06.07.2012 8:29 PM PDT]
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True Sith are simply evil beings. The 'Sith' or Dark Jedi can be peaceful, but the inner conflict makes it more likely that they are evil. Combine this with the fact that there are only two, and therefor the apprentice is chosen very carefully, you'll get evil as result. Everything else is merely a matter of point of view. Why the rebel alliance? Was the Empire that bad?