I would say [member="Darth Arcis"] hit the nail on the head.
There's nothing inherently evil in the Sith code and ideology. Just like there's nothing inherently evil in a Communist Dictatorship. Hell if a Communist Dictatorship worked it would theoretically be the most benevolent form of government there is. Sith ideology however, focuses around gathering power at the expense of others, and as they say, power corrupts.
Peace is a lie, 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.
Look, this isn't hard to read. They ain't fucking around here. This is an aggressive, self-interested, potentially violent doctrine to follow. It is not inherently evil or wrong, but those things are all but inevitable given its writing. In contrast, the Jedi Code is very scholarly and focuses on learning and listening. It emphasizes control and a measured response. Beyond everything else, the two are definitely opposed, and that's going to color the character of the organization.
So Sith are generally selfish, crave power, despise limits and rules (at least rules that control them) and seek victory in all things. So yeah, not inherently evil, but if you know anything about human nature (and lets be real here, there's almost no really alien aliens in Star Wars) you know that 99.99% of the time, that means the Sith are gonna do evil things.
I agree with [member="Ordo"] here. They're obviously meant to be evil. That doesn't mean they can't be great characters or fun to roleplay. Who doesn't like to play the bad guy some times?