What if after Sanji reveals his conquerors, Killingham then pulls out Conquerors and the fight continues on, would that do anything for you?
I definitely don't think Killingham has or should have conqueror's lol. I'm hoping for a cool Longinus technique that would be countered by Sanji in their final clash.
Or are you looking for Backstory?
Eh, it's perfectly fine that both Sommers and Killingham have no backstories, it's not a necessary criteria for memorable henchman-type opponents at all. What I was hoping for with Killingham was characterization that was more uniquely pronounced that standard celestial dragon notions. His racism towards Jinbei for instance felt flat because it's to be expected. Sommers meanwhile had a distinctly nasty edge that Oda had fun depicting (until he flanderized him at least).
His developing dynamic with Sanji is a little endearing, I'll give him that. Hopefully Oda capitalizes on it in the little remaining screen time Kil has.