You do realize that Vrook was the one stating that, and that Vrook sees everyone as evil if they stray from the Jedi path. Even seeing Zayne as evil in the KotOR comics, when he wasn't.
Vrook was constantly under the opinion the Exile was evil even when the Exile clearly wasn't. He immediantly accused the Exile of having already fallen then when Kreia enters the mix he says that the she'll cause you to fall just as Revan did. So in his mind no matter what the Exile is evil.
The Jedi Council was afraid of what had happened to the Exile and instead of trying to understand it, they exiled him/her.
Kreia stated that all the Jedi at Malachor V either fell to the darkside or died, save one. The Exile, how do we know there wasn't another Jedi present that also fell to the darkside but also left a wound in the force similar to the exile and unlike the exile began feeding on people with the force.
If one survived by creating a wound in the Force, isn't it possible someone else did as well.
In a way Nihilus and the Exile were kindred spirits, but they were not a part of each other. The Exile chose one path, while Nihilus chose another path. Nihilus sensed the exile after the force was reawakened and the exile went to one side of the spectrum or the other, however Nihilus couldn't determine an accurate location. One could even argue he may not have even felt it when Visa told him about it, which is why he reacted so violently towards her and why he demanded she bring whatever it was before him.