Disclaimer - I'm drunk.
We talk about this in my room a lot because I am dork and a big supporter of the Sith and a sympathizer with the efforts of the Empire. I refuse to watch or read any spoilers about the new movie because I'm a cinematic purist and I like to go into movies having absolutely nothing spoiled so I will do my best to explain my point of view with zero perspective on the new film and will try to overlook spoilers as this thread continues.
In my eyes, the Jedi are the real-world equivalent of something close to militarized Zen Buddhists (while being, for the most part, extremely hypocritical in their practices) and the Sith are more related to what Christianity has morphed into "Satanism" which was formed from the roots of many ancient "Pagan" religions. Jedi teach strict discipline, abstinence, a detachment from the self and emotions, knowledge of the Force and use of it only in circumstances where it benefits intelligent life as a whole. Very few Jedi actually practice this virtue in the films, many (including notable members of the Jedi Council such as Yoda and Mace Windu) actually commit what could easily be perceived as acts of murder or rage that were fueled by a sense of their order being "superior" or the "right way". This completely contradicts their teachings.
I come very close to saying that Obi-Wan is one of the few Jedi Masters that actually follows the "Jedi Code" until he utters the line "Only a Sith deals in absolutes." That line reverberates for me SO deeply in how hypocritical and arrogant the Jedi are. Blatantly using an absolute statement to accuse Sith of being the only ones that deal in absolutes...
To elaborate on this point, you can't be a Jedi killing tens if not hundreds of living beings with the intent of saving others without being emotionally/morally involved. This emotional attachment is the ever looming "path to the Dark side" that every Jedi shuns and punishes each other for, yet they ALL do it. Every Jedi Master portrayed in the films had committed some form of violence or passionate aggression towards another living being. In fact, the way they defend their actions as being for the love of other beings in itself is a path towards the Dark side, which I will get into later.
Fuck it, I'll get into it now. The Dark side has no rules. The Dark side represents unrestricted access in ALL aspects of emotion and mortal potential as sources of power: love, hate, lust, envy, greed, gluttony, philanthropy, ALL of it. While the Dark side can be consuming to those who lack discipline (which the Jedi are zealots for), it can be used for any purpose the Sith chooses. One of my favorite scenes in the entire saga so far is the opera scene where Palpatine tells Anakin the story of Darth Plagueis.
The only reason the Sith are shunned is because we are led into the story from the perspective that the Jedi are right and are the "good guys". That "Light" is good and "Dark" is bad. There are no shades of gray in this... and the Jedi (who claim to not deal in absolutes) are the only ones claiming "Light is good, Dark is bad", "do or do not, there is no try", etc. From the very beginning, we see Darth Vader and all associated with him portrayed as evil, oppressive figures trying to stop the noble minority who are fighting to take down an oppressive regime. We see this from the perspective of a guerrilla faction of Rebels led strongly by the surviving Masters and aptly named "Knights" of the former religious regime. We do not see this from the perspective of the Empire or the Sith.
I am not going to go so far as to say Palpatine/Sidious was a good guy, he was the epitome of a corrupt politician that used "unnatural" power to become nothing shy of a galactic dictator. Was this the fault of what would later become the Empire? Despite his pulling of strings, the Galactic Senate *elected* him into power. They democratically *chose* the Empire over the Republic. The Empire, as a concept, is nothing inherently bad and Imperial soldiers and workers were not inherently bad people, they were simply supporters of uniting a galaxy, most unaware of the horrific acts that the Sith Lord in command was ordering many of them to commit.
If we watched original trilogy from the perspective of the Empire and the Sith, it's a story of a powerful leader and his apprentice attempting to maintain peace and order through a galaxy in turmoil. The remnants of the former religious regime come out of hiding to unite a rebellious uprising that performs several extremely effective covert strikes against the Empire that, in contemporary American terminology, we would call nothing short of religiously-charged "terrorist attacks".
The attacks are effective. The Rebellion grows. The Empire struggles to recuperate from severe losses of innocent lives, leadership and technology. They invest heavily in suppressing this uprising and rebuilding their army to compensate for their losses since a war has obviously been initiated. A full-scale war begins. Severe casualties on both sides occur. The leader is betrayed by his Apprentice (as seems to be Sith tradition, the Apprentice overthrowing the Master in a fit of passion, in this case love for his son rather than hate or greed... still Sith, still Dark side, using powerful emotions to fuel and empower yourself). The Apprentice dies. The Empire is left with no official leadership or order short of submitting to the religious extremists' violent demands and allowing them control of the entire galaxy.
...
But Ewoks are cute and play drums on dead soldiers helmets... so that's a better story, right?
In my opinion, the Jedi have good intentions (in their eyes) but their stubborn arrogance and hypocritical tenets led them to their inevitable demise. If the potential power that the Sith indulge in were used in tandem with the self-discipline of the Jedi, corruption could potentially be reduced as a "dark" factor in the Force and Force-users could actually have the opportunity to use it's full potential to benefit the entire galaxy... but closed-mindedness on both sides of the Force prevents this, turning the power of the Force into a battlefield rather than a source of benefit. Jedi are too caught up on moral sanctity, arrogance, chastity and "good vs. evil" and the Sith (for the most part) are too caught up in self-indulgence and greed.
I hope at least some of that made sense.