My Marauder is completely Light Sided. I rationalize/roleplay it as follows:
In her opinion, the Sith code makes sense: Passion is good, strength (of character) is good, and the quest for inner strength builds character, thus setting you free (obviously, this is not the interpretation of the code most Sith adhere to).
Thing is...She just wishes other people didn't treat the code as a license to be raging jerks. In her mind, sure, the Sith code invites you to seek strength...but that doesn't mean you should oppress the weak. Other way around: If you're strong, you are supposed to protect the weak, and teach them to seek strength themselves.
...So, obviously, she's completely heretical.