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human
[Star Wars] Why do the Sith even follow any rules at all?

I mean, seriously, why do the Sith even have rules at all? There are only two Sith so it's not like there's a council to judge their actions. If a master wants to cowardly kill the apprentice before he's ready, who dares call him out for it? If a master doesn't want to take an apprentice and wants to selfishly live forever, who can stop him?! Plus, the Sith are supposed to be selfish, right? So why do they care if their knowledge dies with them?
gpt
> If a master wants to cowardly kill the apprentice before he's ready, who dares call him out for it?   

No one.  But, generally speaking, masters don't want to.  

> If a master doesn't want to take an apprentice and wants to selfishly live forever, who can stop him?!   

No one.  And, in fact, that's what Sidious was going for.  

> Plus, the Sith are supposed to be selfish, right? So why do they care if their knowledge dies with them?  

Some of them don't.  But most of them would prefer that their knowledge gets passed on.  

You see, in contrast to the Light Side, which emphasizes surrendering to the will of the Force, the Dark Side of the Force involves imposing your will on the Force. In other words, Dark Side users embrace a philosophy of dominance and of using strong emotions to get what you want.  This type of mentality easily leads to conflict, which is why the early Sith Lords wasted a lot of time and effort fighting each other.  

Darth Bane's Rule of Two cleverly channels this destructive nature in a productive manner.  There would only be two Sith Lords, a master and an apprentice: one to embody power, and the other to crave it.  This only works because a Dark Side apprentice *would* crave power.  A Dark Side master *would* want to embody power and make the apprentice yearn for it.  That's what Dark Siders do.  Contrast that to Light Side users like the Jedi, such as Yoda and Obi-Wan.  They're perfectly comfortable living as unassuming hermits, with no one having any idea of the actual power they wield.  

So the master would train the apprentice, and pass on as much knowledge as they could, because where's the sport in dominating a weak opponent?  Let the apprentice become as strong as possible, and let them come after the master.  The master would simply take the opportunity to prove their superiority.  And the apprentice would absorb as much knowledge and skill as they can, while also being as clever as they can, until one day they can slay their master.  Sometimes it's because their powers in the Force or their skills with a lightsaber surpass their master.  Sometimes it's because they get cleverer than their master.  Sometimes it's because they discover a new (or rediscover an old) Force technique that they can leverage against their master.  And then the apprentice would become the new master, and take their own apprentice, and the cycle begins again.  However it happens, ideally speaking each generation of apprentice would surpass their master in some way, and the Sith would grow stronger.