But there is still the possibility that Revan havent' fall to the darks side corruption, even with the star forge.
Kreia said that Revan NEVER fall, and that he has always been himself, so maybe, if Malak haven't betrayed him, he would been able to make a strong empire and prepare it to foght the true siths. The republic in TSL is crumbling, because of the jedi civil war, and the katarr events, the siths, all of that, and also the incidents caused by malak.
So if Revan really never fell into the dark side, Malak really screwed up everything, so when Revan recovers the memories about the true siths, it's too late for his old plans, so he asked canderous, to unite the mandalorians to help against the siths, and carth too, bla bla, to prepare the republic against the true siths. After this he leave to or try to stop himself the true siths, knowing that if he fails his companions would be preparing the republic, or he tries to retard them, to let more time to the republic.
That's how i see it ^^