Ah, Legacy of the Force. Must you make me rant about you at every opportunity on multiple sites?
I have to vehemently agree that LOTF was a giant waste of paper and potentially good characters. Now, a large part of that is my being a Jacen fan, and I can't stand how badly he was written. Don't get me wrong, the him falling to the Dark Side thing isn't necessarily a bad idea, it was just very poorly written and handled. It does make sense, a little, when you take into account Vergere's idea that the Force was one and that it was personal moral flaws that caused people to fall, rather than an addiction to the Dark Side. Indeed, before his contact with Vergere, Jacen's more paralyzed by fear of the Dark Side and terrible consequences tha either of his siblings. It's not until Vergere teaches him the "Choose and Act" and "Be the Gardener" ideas that he starts to think there's a way around the orthodox idea. So, the basic idea makes sense, and combined with a fear of losing his secret family(ironic echoes of Anakin not entirely bad in this case) should have turned him into a more Lawful Evil type of person to try and head off the conflict.
They should have made him harsh, but with the paramount goal of order, not senseless villainy like burning Kashyyk. We should have seen those good intentions gradually slip away with the Dark Side, proving his view of the Force wrong(I don't actually like that idea, but the majority seems to). Above all, make him competent and an actual threat when the Jedi turn against him, or, hell, bring some Jedi to follow him as well considering his influence after the Vong War. No getting pantsed by illusion Luke starfighter or the Mandos. Finally, don't have everyone immediately turn their backs on him as "too far gone" like they did early in the actual series, he's far better than Vader at the time and he's apparently beyond redemption? Really? Basically, it should have been a reminder of the evils of the Dark Side can turn even the best after so long fighting an enemy outside of the Force, a terrible reminder of the Jedi's own vulnerability. I'll leave more detailed plotlines to other people.
Exactly. He should have been shown as sympathetic and slowly losing control, not immediately cackling and killing everyone who dares oppose him.