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Well, I just finally reached the end of the Abeloth battle in Apocalypse and suddenly one sentence makes all of LOTF and FOTJ make complete sense. I never really understood why Jacen fell like he did until we finally got the full explaination of his vision.
It one of the only times, in fact possibly THE only time, when I've actually found myself sitting back and staring at a page thinking "wow."
And it makes perfect sense for motivation. He allowed himself to be consumed by the dark side in order to prevent his daughter from becoming so. Makes you wonder how truely "dark" he was.