There are two things you need to know going into this topic. A) The Sith were a fictional cult who worshiped a mystical force, which in their universe, granted them crazy telepathic and telekinetic powers. B)This doesn't actually work.
The entire history of the Sith can be summed up as moody politicians trying to control things, and blow shit up. The most well known Sith Lords in the galaxy(This is their preffered mode of address) were:
A)Whiny Mc'Killedmywife(AKA Darth Vader)
And B)The mascot for every aging cream ever invented.(AKA Darth Sidious)
On the surface, these two were close like two peas in a pod. When Vader, who's real name is Anakin Skywalker, by the way, killed his wife and got burnt to a crispy fry, who was there to comfort him? Good ol' Sidious, who was also known as Emperor Palpatine. They then ruled the galaxy, and everyone was happy, and there were unending supplies of space-unicorns and rainbows.
This would have been great BUT...
The sith are backstabbing, evil bastards.
Well simpleton, here comes the answer.
All throughout their history, the Sith Lord's favourite past-time has been pin the laser sword in the other guy, namely, their friends, relatives and mentors. Some very respectable institutions would call this sociopathy, patricide, regicide, or just about anything with a c in it. The Sith Order on the other hand, has no problems with it.
So coupled with this obsession with cutting your loved ones to bits, also comes a neatly packaged sense of anger. And we're not talking normal, "kick your dog because you tripped over it in the night" anger, we're talking "put the animal on a crucifix and bathe in it's blood" kind of anger. The kind of anger that, when left to simmer over the course of 1000 years, tends to result in the deaths of several billion people.
Eh, a few billion people, to eliminate the Jedi Order, which at any given time, didn't number over three thousand Knights. Seems like acceptable losses, right?
If you answered yes, please check yourself into the nearest mental health institution. Anyways, where this is all leading, is that dear old Lord Of The Universe Palpatine used Anakin Skywalker as an over emotional tool for his evil, dark side ends. He somehow managed to convince Skywalker that turning on what has essentially been your family for 13 years is a good idea. Anakin then goes on a rampage, righteously killing every Jedi in sight, and performing crazy acrobatics for the little ones, before cutting their heads off. Later, however, his old mentor Obi-wan caught up with him, and in the process of trying to ram a lightsaber down Obi-wan's throat, he gets cut into several pieces, kills his wife, and whines about how unfair the universe is.
Palps, of course, was there to save little Anakin from being Aniburgers, and not only had a new name to offer the confused, limbless and very angry ex-Jedi, he also had a handy dandy half robotic life support suit just lying around in the attic.
Anakin, who was rechristened Darth Vader, then became little more than a lapdog (see what I did there?) for the whims and ways of the crazed new Emperor. However, like any good Sith Lord, he also spent the next 30 years, and trillions of taxpayer dollars, plotting to overthrow and gain his vengeance on his new Master.
Luckily that never happens in our world, of course.
He did eventually succeed at getting his vengeance, yet somehow resisted the dark side, by saving his pip-squeek of a Jedi son, so I guess everything worked out great for him in the end... Besides the fact he kind of died.