Quick question; bit grisly

Is there any reason it’s not possible to execute somebody while they’re under a general anaesthetic, so it happens painlessly? If not, is the reason this isn’t ever an option linked to the desire for revenge, i.e. if somebody’s to be executed, they have to suffer in the process? Are there any pro-capital-punishment campaigners who would support painless executions?

Update in the morning: question answered in the comments. Just one of those late-night wonderings 🙂