Heroin overdose should work well.
I've heard about that. Is nitrogen asphyxiation really better than poisons like Zyklon B?
Or any other inert gas. But you are quite right. The notion that killing a person quickly and painlessly is some immense challenge is just ludicrous.
The problem is that the opponents are not really against any particular method. Their just pretend to be as a pose to help them in their fight against any form of capital punishment.
BTW, the guillotine takes about 1/10 second. Works every time.
or an opiate overdose.
Quick and painless.