Destruction of the brain would be the most humane for a unskilled owner. Yes, it's "brutal" but it should also be instant.
CO2 is humane for rodents, but as stated above, doesn't work properly with snakes.
Vet euthanasia ensures it cannot be used for food purposes to anything, so it's a waste and you'd have the high vet bill, and need a good reptile vet who knows how also.
I see lots of people who decry the brutal method because they are squeamish and don't want to admit that it's the fastest method for "at home". But also, we should all be aware that if the wrong person saw it, or knew about it, the owner could come under fire for inhumane killing of a pet, or even be arrested for animal abuse, regardless of the reason the snake was being put down.
There's a lot of details to look at overall and of course, there's rarely a perfect solution to a situation none of us would want to be in. As far as me personally, I would rather discuss it and think about it BEFORE I have need of a solution. Otherwise if the situation arises, there might not be a lot of time and the snake could be suffering while I dither over it.
Edit: I just had the slightly amusing thought that if someone kills a pet snake with a hammer, they're a inhuman monster abusing a pet.... but if someone kills a wild snake with a hammer, they're a hero saving the local kids from a vicious wild creature. Can't win sometimes!