It depends on the individual Sith. Some realize they are evil, others think the ends justify the means.
Vader thinks he, the Sith, and the Empire are doing good by restoring peace. He also thinks the Jedi are the evil ones:
Anakin/Vader (to Padme): Don't you see, we don't have to run away anymore. I have brought peace to the Republic. I am more powerful than the Chancellor. I can overthrow him, and together you and I can rule the galaxy. Make things the way we want them to be.
...
Anakin/Vader (to Obi-Wan): I see through the lies of the Jedi. I do not fear the dark side as you do. I have brought peace, justice, freedom, and security to my new Empire.
...
Obi-Wan: Anakin, Chancellor Palpatine is evil.
Anakin/Vader: From the Jedi point of view! From my point of view, the Jedi are evil.
Star Wars: Episode III Revenge of the Sith
He acknowledges that he has done evil things, but he thinks the ends justify the means. For example, he seems to think murder is acceptable if done out of "righteousness" or love:
[Vader is thinking about his past] His murder of the younglings in the Jedi Temple, their eyes wide with a fear that only fed his righteous wrath.
...
She stared up at Vader, unafraid. “I hate you and everything you stand for,” she said. “But when I murdered, I murdered out of love.” Vader raised his blade, his breathing loud and steady. When he spoke, his voice was as deep and hollow as a funeral gong. “I know precisely what you mean,” he said, and slashed.
Lords of the Sith, p. 195, 284
On the other hand, Sidious only cares about power. When Anakin attacks Mace Windu for him, Sidious realizes he's won and screams
Power! Unlimited power!
Star Wars: Episode III Revenge of the Sith
as he electrocutes Windu.
He views life as a survival of the fittest, and the strongest survivors deserve power:
“We are, all of us, always being tested, my friend. Tests make us stronger, and strength is power, and power is the point. We must pass all the tests we face.” A long pause, then, “Or die in the effort.”
Lords of the Sith, p. 149
Sidious doesn't really view the world in good vs. evil -- instead he views it as powerful vs. powerless. He does mention good vs. evil while lecturing Anakin on the Sith vs. the Jedi (when he tells Anakin about Darth Plagueis):
Sidious: Good is a point of view, Anakin. The Sith and the Jedi are similar in almost every way, including their quest for greater power.
Anakin: The Sith rely on their passion for their strength. They think inward, only about themselves.
Sidious: And the Jedi don't?
Star Wars: Episode III Revenge of the Sith
From Sidious' perspective, the Jedi and Sith both attempt to selfishly gain power but the Jedi call themselves "good" from their point of view. He thinks "good" is at best subjective. Again, he doesn't really care about good vs. evil -- he's only saying this because Anakin thinks it's important.
He does seem to realize that he's evil (and doesn't care):
[Sidious thinking to himself] As his Empire swelled, bringing more and more of the outer systems into its fold, so too would his power unfurl, until every being in the galaxy was held captive in his dark embrace.
Tarkin, p. 242
"Light" vs. "Dark"
Regarding the Sith use of the name "dark side" of the Force, they think both the light side and dark side of the Force are permissible whereas the Jedi unnecessarily limit themselves to the "light side". This is stated most explicitly in the Legends novel Darth Plagueis:
What separates a Sith from a Jedi is the way each approaches the Force. The Jedi Order has placed limits on itself, but the Sith have never shied from incorporating the power of the dark side to accomplish their goals.
Darth Plagueis, p. 365
Sidious also famously lectured Anakin that
The dark side of the Force is a pathway to many abilities some consider to be unnatural.
Star Wars: Episode III Revenge of the Sith
Sidious thinks power is all that matters, so he has no qualms about using the dark side. Vader thinks the ends justify the means, so the acquisition of dark side abilities is permissible. From the Sith perspective, the Jedi are fools to limit themselves to the "light side" only. The Sith view the sides of the Force more like "defensive" (light) vs. "offensive" (dark), and as they say sometimes the best defense is a good offense. The Sith use the Jedi names for the sides of the Force because that's the common names for them, and they simply don't care that "dark" has evil connotations.
