Destiny is often found on the road one takes to avoid it.
The title character is the son of a Jedi whom you killed during the prologue while learning the controls through Darth Vader. Vader kills several troopers in the presence of the boy and the boy grows up to become Vader's Secret Apprentice.
A series of missions is meant to culminate with Vader and You taking down the Emperor. You'll meet and battle various in game characters. Some from the movies and others new to the game universe. Order 66 survivors, various Alliance personalities, well them before the Alliance.
Some of your compatriots are twisted versions of Saga characters. PROXY for his holo-disguises is a twisted version of 3PO joined HK-47. Juno is like various hard as nails female love interests such as Leia.
At first you'll be killing everyone, but it grows more selective as you become the catalyst for the Rebel Alliance. Likewise you'll discover the truth of a kind about your past, and so you are going to save the Alliance founders.
Essentially its made for you to be the light to show the way despite your past so Galen Marek is more of anti-hero that does things that are heroic.
Likewise the ability to make use of gradually intensified Force powers is awesome.