If Charles ever gets his duel, my gut instinct would not be for avada kedavra for a variety of reasons already mentioned. Also Charles is a giant asshole but he's not got an illegal dark magic fascination or a desire to lose everything and rot in Azkaban.

I'd have thought there would be different styles of duel and duel etiquette and the one you'd go for would depend on the context of the challenge. Charles, for example, would either opt for a low stakes duel if the challenge was made publicly but ideally he'd prefer a very discreet, less gentlemanly sort of aggressive duel but not to the death, more like first to cause injury but it might get heated and go a bit too far.

I could see illegal to the death duels with avada kedavra as the only spell you use being a thing because canon says it won't work if you don't mean it so it wouldn't necessarily be zap and it's over, in fact it'd potentially be very tense even for the caster (unless they're a sociopath). It'd be deeply illegal but dueling being illegal didn't stop people dueling and killing each other. That said I feel like a more popular and more palatable option might be diffindo each other until you're sufficiently bloodied and regretting your life choices. Sometimes amputation(s) or death occur. Oops.