All the advances in execution methods haven’t been made to make it more humane to the victim - they’ve been made so it seems more humane to everyone else.
AFAIK, statistics-wise, the execution method with the lowest quota of horrible mishaps is the guillotine. A sufficiently fast 4t weight to the head would probably be even quicker for the brain to go, although it’d also require more cleanup.
(Yes, even overdosing on narcotics has more mishaps - and there are little to no narcotics abailable for executions, because the producers don’t want them to be used for that.)
All of the more reliable methods are… grisly, and civilisation doesn’t want grisly. We want to press a button and the victim goes to sleep to never wake up, because that makes it easier on us.