The second-worst Legion Corruption narrative is The Emperor's Children with a bullet, and for much the same reasons: it immediately becomes 66% more compelling if you just ditch the fucking sword. Let Fulgrim own it.

Emphasize Fulgrim's desire for perfection and accomplishment, and put it at odds with his Legion's low numbers and poor recruitment rate. As the Crusade grinds on he's looking for any edge, any marginal gains. He can't be left behind, he's FULGRIM!

What drugs, what sensory enhancements, what new organs and tweaked nerves and hypnotic processes, what will let his Legion rise above and achieve everything they deserve to? Dabbling in daemon stuff should just be the next level of steroids, to him. It's a new ascension.

The Laer don't corrupt him by leaving behind a Sword Of +1 Evil, they corrupt him by giving him the chance to loot their genetech. The Emperor puts it under lock and key, but... hm, maybe just one little tweak? Fabius knows what he's doing. Why would the Emperor deny him this?

As Magnus is to occult lore, Fabius should be to the whole Astartes enhancement process. He dances on the edge of going too far, but he's certain that he has the Emperor's tacit approval and understands him better on this matter than anyone else.

He's desperate for that SSS-rank no casualty 100% accuracy run, on all his missions, all the time. Compared to the bigger Legions it's the only way he can keep up. He personally grooms each marine like a Fire Emblem player shipping their faves, and each loss is a tragic waste.

That's where Fabius' cloning experiments first get their legs. Fulgrim loses a pet project, some Captain he was grooming for a specific role, and a lightbulb flicks on in the Apothecary's labs.

"Hey boss, what if..."

The constant, self-destructive search for more, more profit, less cost, more gains, dovetails wonderfully with Fulgrim's origins, and it's a real shame that the Horus Heresy novels kind of neglect those.

Chemos was a capitalist hellhole that had reduced its atmosphere to a polluted nightmare. All that remained of its population were a series of corporate enclaves sustained solely by constant, desperate scavenging. Its leaders were called "Executives", for god's sake.

Fulgrim leveraged his skills to reshape Chemos into a paradise world where everyone could live lives of beauty and leisure and self-actualization, free from the rat race... then fell into the very trap he'd helped his people escape.

Chemos also opens up an interesting avenue for groundwork. Fulgrim worked to enhance the efficiency of Chemos' recycling machines, giving his people more to work with. What if this led him to encourage enhancements that increase the efficiency of those same people?

All that said, I'll give credit to Fulgrim's character for this being a pretty natural interpretation once you cut the Evil Sword or Evil Painting or whatever it is this week. The Death Guard require more fundamental work.