There are several challenges that I see in order to make a story of this sort work for me.
First, other people would need to be brought into the struggle. There's only so much internal angsting I can take before I just plain get frustrated or bored with it. Each new person encountered brings a new flavor, a new problem, a new hope, a new fear. And each one should change him, even if only a little. If your MC is going to be the "plot" (at least to some extent) then he needs to move forward.
One way to do this is to have him become aware of the monster inside him as a separate person, one who becomes a character in its own right. Has this been done before? Absolutely. Many times. But, as the Book says, there is nothing new under the sun. Find your own twist.
Another way to do this is involve the people around him in his conflict. Perhaps he has a friend or lover who is a really good person, and who gives him a reason to struggle against the darker urges. He might have another friend who he finds to share the darker parts of himself (the Dexter series is a good example of this pattern). Maybe the story is about him deciding which path he will allow himself to be taken down. There is the potential for a really gripping crisis point when the two influences in his life collide and he is forced to make a choice.
Maybe it's about him learning how to use what is inside of him for Good. There's an awful power in being a monster. I've read statistics that say a statistically significant percentage of successful CEOs are sociopaths and psychopaths. The reason for their success is that they are more likely to do the bad things that get you ahead financially but which people with more ethics would refuse to do. It makes sense. Your MC could harness that power to accomplish something most people wouldn't be able to do.
Which leads me to the second big challenge which I see with this story. If I start reading a story about a MC who has a demon inside of him, I would need to see redeeming characteristics that assure me that I will not be emotionally investing in a character who will "cross the line" and do something that I cannot tolerate. Dexter does that beautifully, IMHO, at least as of season 5. He does horrible things, but only to horrible people. By some standards he does good things, even if he does them in horrible ways.
Many people cannot tolerate Dexter because, for them, he crosses the line. The "lower" your line gets the more people you will lose. If you want your book to sell, it needs to appeal to the largest number of potential readers possible. Decide where to draw the line, and show readers, fairly early, an example of your MC drawing that line. That way, potential readers will be able to decide whether they fall above or below the story's threshold for ethical behavior.
Ask yourself this: what, in this story, will make people want to be inside my MC's head? People read fiction to experience things vicariously which they would otherwise not be able to experience. What positive experience will your book offer them? Once you have decided that, you have your set-up. Give them hints of what is to come, and they will be drawn in. If the draw is to feel powerful, have the MC start out by doing something powerful. If the draw is to escape a horrible life/situation, show us how horrible his life is and then immediately show us how he is trying to escape it. If the payoff at the end of the story is going to be that he destroys some horrible organization (perhaps the cult that "created" him) start us out with him encountering the organization's members or effects and opposing them.
One key point to think about is that what you care about in the story is not what readers are going to care about, unless you are able to bring them round to do so. Obviously the internal conflict is fascinating to you, and I understand the draw. The internal workings of my own characters fascinate me. I know them completely. I know exactly how they would react to any situation, and why. Their background is all mapped out in my head. But background is not the story, and highlighting the turmoil may not be the win.
So your character is conflicted. Why should I, the reader, care? Until I care about your character, his turmoil is just background noise. In order to make me care, you need to show me why he's a person who I want to cheer for. Show me his virtues and how he uses them to accomplish things which are good. Then I will care about his internal conflict and I will keep reading to see it resolved. If I feel he's a person who deserves the win at the end of the book, I will be willing to wade through the Bad Stuff in order to see that win happen.
Even if he's not a particularly virtuous person at the beginning of the book, show me that he has a potential which is 1) strong enough that it is worth wading through the angst and tension to get to, and 2) that you, the author, are going to lead me toward that potential. If you can convince me of both these things early on (certainly by the end of the first chapter), I'll read your book.