the answer to the 'why' that keeps popping up:
because our country is too damned politically correct and has become soft due to all the bullshit legal cases that pop up at the slightest bit of 'ethical mistreatment'.
my way: kill the offender the same damned way they killed the victim. now, of course the death penalty would then only be used when it can be proven beyond any doubt that the offender is guilty. Many murder cases are pretty much that way, however some are a little more difficult. That's when you can use life in prison or something, but the death penalty shouldn't be pain free, but it can still be nice and cheap: maybe a bullet one day, gas the next, knife the next, and maybe finish the week with a chemical.
but I'm all for capital punishment and straight up execution.
As far ways that are both ethical and cheap, I agree there are many other methods possible, but I suspect the current standard is to prevent all ethical treatment cases. Using anesthesia and then CO could be seen as unethical by some greedy scumbag family members 'because it starved him of the oxygen he needed, not killed him in a painless manner'. The ethical treatment is far from serving the wishes of the criminal, but rather serving the state by protecting it from the ravenous members of the family out for money.