That was precisely my problem with the Legacy series it was a bad rehash of the prequels. Jacen basically goes through the prequel Episode III Anakin route and hurts and kills people who love him.

He falls the same way as those before him fell, instead of needing power to save his wife he needs the darkside for control and to bring peace and stability to a galaxy forever at war, only sometimes does peace prevail.

He wants for selfish reasons the galaxy to be a safe place for his child to grow up in. He is willing to sacrifice anything to achieve his ends. He is believable in that instance. Parents will do anything to protect their children from harm.

Even though he merge into the force rather than becoming one with it. I still think he was not beyond redemption his last thoughts and wishes were to protect his daughter, though Jaina did not believe him.

The jedi were so changed and darkened by the Yuuzhan Vong war as to be perilously close to the darkside. Even Luke accepted Vergeres ways at first and then later rejected them. Even now Luke does not know vergere was a sith lord. Mara probably suspected, and Jacen knew as well as Lumiya. They are all croaked now though,lol.

You also feel bad for Tahiri who lost Anakin and through her love was twisted dark by Jacen. Through Ben Skywalker's intervention she gets a chance at rememption.