Jacen Solo, son of Han and Leia in the EU novel series. Jacen's story is simply heartbreaking and how he met his end is even more saddening. He and his sister Jaina, twins, had started out as Jedi until Jacen learned to Flow Walk (a forbidden Force technique of seeing the Future) and after being trained by another Sith women. He see's a vision of horrible things and his young daughter being harmed. Basically, the only way he thought he could prevent the horrid future and potential destruction of the galaxy was to become a Sith Lord. He adopted the name Darth Caedus and broke away from the Jedi and started running a band of baddies.
During this time, his sister Jaina became one of the best warriors in the Jedi Order and Jacen's daughter Allana Solo, who was now roughly 8 or 9 starts having Force dreams as well. Jacen does a lot of bad things and the Jedi try to stop him. They catch him and the only one around able to really match Jacen's insane Sith power was Jaina. So, just before Jaina catches him, Caedus see's another vision of his daughter caught in an explosion that would happen in in a few moments, and while trying to run through passages underground and through tunnels to his ship kind of a long run away, Jaina catches up with him. They fight as they run down the tunnels side by side and Jaina ends up running the saber through his chest. All Jacen was trying to do was save Allana and the entire galaxy.
You learn later that Jacen accidentally freed a being far worse than Palpatine, her name was Abeloth. She was horrific and far worse than any Force user in history. By flow walking and seeing the future, Jacen tried to stop the future events and in doing to, altering the time line was one of a few things that could free Abeloth from her prison.
Truly, it stuck with me ever since Fate of the Jedi was released. It is the most tragic and sad thing imaginable, knowing that you must become the right hand of death and slay innocents by the thousands to insure the safety of trillions, do try to save your daughter at all costs and to be killed in route by your sister. That is heavy...and affected me greatly.