Actually, Jaster, I achieved that exact thing you're talking about with Antoine. Although it appears that he a) followed his nature and b) eventually felt remorse for it, the reality is that all of his malice and cruelty - everything that makes him a demon - is held in and multiplied thoroughly by Malignitias. True, time helped, but there is a dynamic there, and I achieved it by isolating his malice into an object. There's nothing to say you can't use that example yourself.

For instance, you don't need a weapon and you definitely don't need to make it as powerful as being immune to purity. Instead, you could make the ability correlate with a plethora of possible themes. You also don't necessarily need an object. You could create a dynamic, such as being a farseer, by manifesting a third eye to see into the future or something like that, and do so by isolating your demonic nature into one spot. What this would do is give your character time to think and feel without the presence of their demonic nature, and over time - far less than the 1,700 years it took Antoine - they could become curious to their existence beyond their genetic nature.

I would note that a dynamic like that is entirely based off my character, so you could format it how you see fit. Regardless of how you do it, there is a character with that sort've dynamic already existing in the rolpelay. Essentially, there is already a Big Bad Seeking Redemption. 

On that note, vampires aren't much better than demons. If you spliced them together, you'd still have a giant Dickasaurus. Cruel, vindictive, self-centered, etc.

What you're looking for is a demon looking to be derailed from being a psychotic killer, and in that very essence, you're looking to make an individual go astray from the nature of their race. I think the answer you're looking for is more deeply integrated in a plot and psychology instead of a huge physiological change. No matter what, the dynamic is still going to be prevalent in your character because no matter how well-written or avoided it is, it will be existential for their entity.