Originally Posted by SilentScope001
It is merely the Dark Side that the Exile rejected at Malachor V that Nihilus becomes.
This is my take on the matter as well. It somehow seems incorrect or insufficient to say that the exile merely rejected the dark side at Malachor V, since the choice to turn to the dark side is still there. It would be more correct to say that the exile rejected being dictated to by the force and shed himself of anything that would force the issue upon him. My theory is then, that what the exile shed himself of becomes Nihilus. But since the choice of Malachor V was to either die or turn to the dark side, then what survives must be definition be dark sided. The exile himself will not make a choice, so he remains grey and can freely turn either to the light or the dark side at a later time.