How can you complete the game on light side or dark side in star wars the force unleashed?

The real question is how do you complete the game on the dark side.

Are there any shortcuts.How can you complete the game on light side or dark side in star wars the force unleashed?I don't know if the game's different on XBOX360 (coz my console got the circles of death again!) but on ps2 the storyline is set and you end up turning your back on the dark side.