I don't cull my chicks until they are eating age, I hatch them as replacements and to eat, male and female. And I only hatch as many as I can eat, so my goals are different from yours. For young chicks with obvious defects that affect quality of life or their ability to grow and thrive poultry shears to the neck work well. But at any age any method you can manage that causes instant death and not wounds and suffering can work. To me the harder question is how do you dispose of the bodies, especially when they are older and bigger. I live in the country and have a spot I take carcasses where wild animals can eat them, whether chickens or critters I trap like raccoons, skunks, or raccoons. Not everyone has a good disposal method.
I've seen other breeders of show quality chickens with that same rational, they don't want inferior chicks out there, especially if they are breeding champion chickens. They don't want inferior birds associated with their line.
But think about your rational a bit. Who are you giving or selling them to? Other people trying to breed champion chickens? Or is it to people that just want chickens? I'd venture to guess that the vast majority of people you are giving or selling them to don't care about the SOP and are not trying to breed to improve the breed. I imagine you are selecting for dark eggs, not all Marans breeders do that. Even your culls probably look pretty darn good.
Of course it is your choice, but I just don't see how selling or giving away your culls is hurting the breed. If one of your customers is interested in breeding to the SOP then talk to them about your quality birds if you have any you are willing to part with.
A technical point, cull does not mean kill. Some general definitions of cull include select, choose, pick, take, obtain, or glean. You are already culling your birds by selecting which ones you allow to breed and which ones you sell or give away.
With the number of chicks you are probably hatching to get enough to select quality birds I certainly see that you cannot afford to raise them all to butchering age, plus just how many chickens can you eat in a year. If you are serious about breeding a show quality flock it's a legitimate issue, what do you do with the excess.