In addition to the quick painless methods listed above the doomed prisoner could be put under with conventional anesthesia and never know it’s coming. If the public wants a spectacle just use a guillotine. Drugs are highly unreliable but there’s no real compromise between the factions that want executions to be quick and painless and those that desire suffering. The condemned could also be given the convenient means to do themselves in but the political problems are even worse. Carbon monoxide could be used, possibly in combination with nitrogen as well.