Sooo, you don't know that in fact Darth Vader was never a sith lord ?

Vader was fooled by Palpatine to join him, with the promise that he could keep Amidala alive.

Once he realized the trap, his plan was all along to destroy Sith and Jedi (because both sides lied him) and bring the balance into The Force.

Many people didn't paid attention to all these details in the saga and fall (as usual) for the media bullshit presenting Darth as "the bad guy".

I suggest to all re-watching the whole saga, concentrating on what Darth is saying.