That is exactly what is wrong with the Sith. Even though the following is not only about how evil the Sith are, but also about what is wrong with them. Or people as a whole.
Admitting a thing is impossible is just giving up on trying. Doing something just because 'it's natural' (which it may be) is the worst thing I can portray. Sith may realise peace is not natural, but Jedi try to reach it. They truly want to achieve galactic or even universal peace, they don't give up. Because giving up is a failure. Most Sith I have heard of don't think failure is permitted, so I have come to a whole new question: what do Sith actually want? If their own code tells them to admit something is near impossible, it's a failure. But if it's a failure, we're not talking about Sith.
Peace is an utopia. Many people seek it, because utopia is perfect, it's good. Even though utopia is really-really unlikely to happen due to nature, as some speak, people still try to work towards it. Then there are the ones who say they're realists. They say something is impossible and use it as a reason for everything. I may be the only one who doesn't think this kind of behaviour is acceptable. But it doesn't matter, everybody has their own opinion.
So, I'm continuing. If Sith really were good, then why would they not try to achieve a perfect universe? Trying to reach a state of better life is something which is never illogical if the right ways are used. So why should one even admit something is meant to be and what others seek for is impossible? Why would one try to ignore the fact that if everybody did something, utopia could be reached?
These are Sith. The ones who say peace is a lie.
It is a lie. As long as there are Sith.
Sith are evil.
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I shouldn't bring philosophy in anymore. At least not in English. I don't even want to know how illogical my text may be. I hope it's fine, though.
NOTE: I appreciate everybody's opinion and just like Jay said, most likely this question doesn't have an answer. So I am just presenting my opinion, maybe even debating a bit.