Use what hospitals use for anesthesia, count back from 100 and by 97 your out like a light. Just keep giving until dead.
1 drug needed...why they use 3 to kill someone is stupid and politically motivated...Hell, have a vet come in and do it...3 seconds and the dog is peacefully put down, they just keep giving the med until the syringe is empty....better than those that get the death penalty deserve...
THere is some silly convention that says you cannot execute someone unless they are cognizant that you are executing them. So the combo drugs keep the victim alive so they die awake or some such BS.