The "taint" came from Malachor V, where Revan learned of the true Sith in the Trayus Academy. It's unclear how far Revan had fallen from the light at this point, but he was determined in his efforts against the Mandalorians. Perhaps he had already fallen at this point. Or perhaps it was a sacrifice, as Kreia suggests. Which is true is for the player to decide for himself or herself.
As I see it, Revan either fell to the corrupt teachings of the Trayus Academy even as he learned of the true Sith and decided he would need to fight them, since they would present a threat to his own emerging empire, or else Revan realised that the true Sith were a threat to the republic that he needed to stop even at the cost of his own soul. Kreia suggests the latter, but we really don't know. Personally, I think it's a little of both - Revan was still benevolent enough to do it for the sake of the republic, but he made the same mistake that Ulic Qel-Droma did - he thought he could wield the dark side as a tool or weapon while avoiding its corruption. And he thought he could do it alone, which is always a mistake for a jedi - the jedi are never alone! Being alone is the mark of the Sith, since there can be no trust (or friendship) among the evil and corrupted.
If so, then he erred, for after defeating the Mandalorians and claiming the StarForge, Revan's next war was not against the true Sith but against the republic - his intent to embrace the dark side in order to save the republic had corrupted him, and instead of protecting the republic, it became simply another obstacle he had to subdue, so that his own forces could grow stronger for the subsequent war against the true Sith.
But then something unexpected happened. A group of jedi infiltrated Revan's ship, and Malak used the opportunity to betray him. Revan survived, but lost his memories and was captured by the jedi, who reprogrammed his memory, as we saw in KotOR1.
Last edited by Jediphile; 09-01-2006 at 11:03 PM.