Huh. This old chestnut popped up again, primarily because I'm awake at the lousier hours of the morning.
To be frank, the Sith code encourages selfishness above all else.
Peace is a li
e, there is only passion.
Through passion, I gain strength.
Through strength, I gain power.
Through power, I gain victory.
Through victory, my chains are broken.
The Force shall free me.
Note the language here. All first-person. All about self-empowerment, and self-support. Taking what you want. Following your intent--whatever that may be--and being victorious in it. As a mantra, it's entirely centered on the self.
Now let's look at the Jedi code (Odann-urr's version) for comparison:
There is no emotion, there is peace.
There is no ignorance, there is knowledge.
There is no passion, there is serenity.
There is no chaos, there is harmony. (this line is edited out at times, but is not considered apocryphal)
There is no death, there is the Force.
Well, to be frank, this whole discussion feels more about law vs chaos than good against evil. Sith are, by their own code, individuals driven by passion. to follow the code is to follow the desire of the self--to do what one wants, and seek freedom above all else. The Jedi, instead, have a more legalistic and rigid mindset. They restrain what they feel, hold back, and restrain their desires for what they believe is the will of the force.
Now, this is where it gets a bit tricky. You see, these codes don't create evil behavior, but both can condone it. So, then, why are sith so commonly found killing and pillaging, wreaking havoc and raising hell? Well, that has to do with another issue altogether--the emotions used in the Force. The "sides" used in the force.
The light side and dark sides of the force are kind of clunky, but mechanically, they are aspects of the force keyed to certain emotional behaviors. The dark side thrives on anger, rage, passion, fear and general hatred. The light side tends to thrive on compassion, mercy, self-sacrifice, and other positive emotions. So the Dark side is corrupting. Okay, that's a thing. Now, can the dark side corrupt non-sith? Well, yes. We've got plenty of dark-siders in the universe who were wizards, sorcerors, adepts, and not Sith in either philosophy or discipline.
This produces another question. A harder question. Can a Sith follow the teachings of the Sith and be a light-sider? Can a Sith practice philosophy of his teachings without the Dark Side?
Theoretically, I'd wager that's a yes. You could be a Sith and not be a Dark-sider. You can wish for your own freedom, use power to free yourself and serve your desires, deny the idea of inner spiritual harmony, and still be a compassionate, benevolent person. Would it make for a strange, almost alien Sith compared to what the standard on this board is? Abso-fraggin-lutely, friendo.
And the weird thing is that it apparently happened before.
Mutiple times.
So, to answer your question of Sith being evil, the answer is a half-hearted one at best: The Dark side is evil. Most Sith follow the Dark Side, but one does not have to follow the Dark Side to be evil. The Dark Side isn't always a Sith thing, and the Stih aren't always darksiders. Traditionally, historically, typically, yes, but, there are exceptions.
My Sith character, Olom Grihk, is not such an exception, but it may exist on the site. It definitely has existed at a few points in canon, apparently. But I haven't hung out with them yet, I think.