If Jedi are like Samurai, what are the Sith?
A philosophical fantasy question and not one anyone may have been asking anyway I agree, but it's what I was contemplating when I looked at my father's day present: a Darth Vader bust cookie jar.... "[Look]... I am your father..'s day present..."
The headgear of Darth Vader is almost certainly based on the the war gear of samurai lords: their kabuto (helmet) and their mempo (face mask). In samurai days, these were both fanciful and practical. They were usually fairly artistically designed for the daimyo (lord) usually to inspire reverence or fear, as well as to indicate clan symbols. The fearful mask therefore is very apt for a Dark Lord of the Sith.
But that brings me back to my question, how would you equate the Sith in samurai terms. First of all, let's get some historical facts right, the samurai did have a strong sense of honor, but this did not generally make them benevolent. In today's terms, you might even call them single-minded and ruthless. Their sense of chivalry is not the idealistic romanticized version of Arthurian legends and European myth. In fact, those ideals are closer to the Jedi; the very clearly defined, although fairly blase, "good" side of the force. Samurai had the right to kill anyone of a lower class (farmers, artisans, merchants) and did not even need to have a motive. They fought the wars against other samurai based on what their lord indicated, and often that meant devastation to the rest of the population. So you can hardly call them similar to the Jedi. By rules of logic that invalidates my original premise but leaves the second part unanswered.
The Sith are the opposite of the "good" forces of the Jedi, bent on qualities we consider evil: absolute domination by force. In Arthurian legend, there generally was not an evil counterpart that was formalized to such a degree. Rather there were individuals like Mordred, who fought against Arthur, and therefore was considered the "evil" power-hungry opposite. So in principle there were equivalent there.
In Japanese historical culture there were three other categories: the ronin, warrior monks, and the ninja. The ronin were simply the "masterless ones", when a samurai lost their lord and hence no longer had someone to pay the bills or give allegiance to. Because there was such strong clan-alignment, it was unlikely you could simply be picked up by another daimyo, and therefore, without anything but their military skill, they often turned pauper or worse (in the old Japanese class system), into farmers or workers for the merchant class. Some did turn to crime but none of it is an institutionalized practice as the Sith model.
The warrior-monks were generally just a separate rank of folks in the clergy and by Zen ideals and pacifism, could hardly be considered "evil" like the Sith.
The ninja are perhaps the closest in a way. They were often paid for hire warriors who did the dirty and dishonorable work like assassinations and mayhem. A better description is perhaps outlaw not in the Jesse James in the the Cowboy-West sense, but as people who made their own rules separate from the rest of the law of the Japanese society (which in truth was very restrictive in history). They were an institution too in the form of ninja schools of thought and practices that trained an army of folks over the years. Yet, the goal of the historical ninja (versus TV-ninja or Internet-Ninja) tended to be more of mercernary intent rather than megalomaniacal domination like the Sith.
Looking back at other historical cultures, the Mongols, Chinese, Romans, English, Persians, and others were certainly bent on world domination (or at least their definition of the "world"), but one person's evil overlords are another's great and wondrous leaders, and vice versa.
So Vader, Sidious, and all Sith going back into the fantasy culture do not have an easy comparable. In fact, in terms of good writing, the black-and-white metaphorical comparison of Sith to Jedi, is quite overly simplistic and trite as concepts. It's fit for teenagers and kids but as you get older you realize there is a lot of grey and even good people do bad things at times, or vice versa, or are labelled bad or good depending upon who you ask.