The "torch" of evil can only be carried by moral agents, without them there can be no evil because there is no moral perspective. That is why good or evil can not exist outside of the perceptions of moral agents, i.e. there is no evil that simply exists and is evil because it's evil on its own.
Well said, and a key observation too frequently ignored by those who seek to objectify evil. The practice of evil can only be explored in the context of an existing moral framework implicitly accepted by the characters, and by the audience through either analogy or suspension of disbelief. Evil can only be experienced by an agent with an established moral compass, and thus can only exist as a judgment rendered by a moral agent on itself or another. In case the agent considers itself evil, the conflict is internal, and psychological. In case the agent considers another evil, the conflict is external, and philosophical. In practice, because of the need to create empathy with the audience, works of fiction usually stick pretty close to the contemporary concept of morality, which then becomes the main guide for what would be considered evil within the work.
These days, it is cliche to advertise a piece of work as exploring moral issues and not painting the world in black and white. What this always amounts to is an interrogation of the modern moral framework. We might expect to be challenged on a moral value we take for granted, to see it situationally subverted, and to ask ourselves whether our internal moral framework conflict with that of society's. This might be done through metaphor, analogy, or just plain reference to actual events in history. We come out of such a experience, when successful, with a more detailed understanding of both where we stand, and where society stands. This is what I expect to see out of Obsidian.
Yet what is rarely explored, and just about never in games, is how moral frameworks came to be - how and why evil got to be such an important idea. Perhaps artists consider the issue of right and wrong too basic, or perhaps they just don't know how to answer it, and leave it to the evolutionary biologists and historians. But this is the more important and more valuable direction of moral exploration - to understand and present, through art, the processes that made us into moral agents; that created the idea of evil.