I started to play Star Wars: The Old Republic. In this game, you can choose between 2 factions (Jedi/Sith). However, in the quest you can choose what you want you do. For example: you can either torture a prisoner (dark side) or let him go free (light side), so you can increase your amount of dark or light points. As an example, my character has 100 light points and 550 dark points; I play as a Sith.
As far I know, it is possible to have a Sith using light side and a Jedi using dark side, without a change of faction.
Why is this? How it is possible to have a Sith using the light side of the Force? Perhaps I'm missing something?