That Luke his love for his friends caused him to stay on the Light side. However that same love turned him to the Dark Side not much later.
The things that are allowed in the NJO books are crazy at the best of times. Its so easy to kill a jedi's partner, child, or the friends or whatever. Even in the times of the Old republic when emotional attachments were not allowed assassins hunted Jedi by taking down those things that they cared about. They used their attachments to turn them to the Dark Side. And these were often minimal attachments, a Dark Jedi of power could easily turn many people to his cause. Far moe then any Dark Jedi or Sith Lord in ancient times could have because it would take less persuasion to do so.