One of the greatest Alignment screw ups is people claiming Vader to be 'Lawful'.
There is nothing Lawful about Vader. He has no code, does not respect family honor or tradition, is bound to no laws, and acts however his rage, hatred and anger dictate (subject only to the will of the Emperor, whom he only serves out of fear).
He openly betrays the Emperor to Luke in ESB (join me and we can overthrow the Emperor and rule the galaxy as father and son), betrays Lando (Im altering the deal, pray I dont alter it any further'), is party to Tarkin lying to Leia when Alderaan is blown up, finally betrays the Emperor himself in RoTJ, attempts to kill his own Son, threatens to turn his own daughter to the Dark Side (or presumably kill her as well), kills his best friend, murders several Imperial officers.
He's a team killing jerk, motivated by his own rage, hatred and fear. An audience with him is as likely to see you throttled to death, as it is for you to get any other kind of response from him.
It's just he's in service of a LE Empire. People conflate the two.
He was Chaotic as a Jedi (reckless, impulsive, headstrong, and acting in defiance of any code). But at least he was also a Good man.
As a Sith, he was just as reckless, headstrong and impulsive. It was just that he was also a morally repugnant monster.