I understand the desire to learn the dark side and become powerful, but why follow "the religion" of the Sith?

For instance, Kreia describes Darth Nihilus as someone who doesn't care about the Sith organization. But, why should he or any other Sith Lord care about anything other than furthering their own ambitions? Why take the time to adhere to the Sith principles? Also, if Darth Nihilus didn't care for the Sith, and only cared about his hunger, then why did it bother his holocron that Darth Krayt chose not to follow the rule of two?

Sidious' main goal was to obtain full control of the galaxy. After killing his master, why did he bother trying to preserve his Sith identity by collecting Sith artifacts and maintaining a hate for the Jedi? I understand he had to get rid of the Jedi because they stood in the way of his goals, but his hate for them seemed to stem from his Sith teachings rather than because they were an obstacle.

What is the motivation of a Sith Lord to keep their Order going?

By the way, I've only watched the movies and read bits and pieces of Wookieepedia. I've never read the EU or played the KotOR games, so forgive me if the answer to this question is obvious.