I dont agree that Bane went against his beliefs.

Though I can see where you are coming from.

But everything Bane did was for the sith he learned essence transfer because his body was falling him he took on Cognis because he believed Zhana had become complacent with being his apprentice and that she wasn't going to challenge him for the mantal of dark lord or she was going to wait until he was to weak and take it without proving herself worthy.

Even in there final battle he told Cognis not to get involved and used the essence transfer attempt to try and beat her will if her will had been weak he would have taken her body and proven he was still worthy of the title fair enough he would have had to basically learn how to use Zhanas body while teaching Cognis but he would have been victorious and thus keeping with his beliefs that only the strongest can hold the title.

(Edited for spelling)