hello. Your decissions are affecting the game play . So dark is opening the possibilites for the darkest alliance members and light for the Oposite ones. It is not mattering, which faction you are in.There even are Sith, which are at the Light side. But if you pick this then companys like the council of fear never will be avaiable for you.
So it is a hard decisson sometimes. Like having to do verry bad stuff (which was in fact happening at the storry anyways) or decide to not but leave your choosen path for it.
I am only refering at 1 of a lot of things: having to decide for let them go or punish 1 by killing his /her beloved one. Like the Sith Imperator did.