I see no reason why murderers sentenced to death are not simply shot in the back of the head with a .22 long rifle. One shot, instant painless death. Cheap too.
Blood splatter and a mess. I know, hanging has its drawbacks too. I tend to be a conservative and hanging has a long tradition. One problem is with the increasing obesity of the criminal class, more and more of them may risk decapitation. There is an Army field manual on hanging that specifies the length of the drop as a function of the convict’s weight to try to ensure a quick death with limited possibility of decapitation. Still, there are ocassional decapitations and instances where the convict is strangled rather than dying instantly from a broken neck. In any case, when people are hanged, they are generally left hanging for a good time to insure that if they are not killed quickly, strangulation will take its toll.
As a practical matter, probably the best method of execution is aphyxiation by nitrogen, which is how animal pounds do it. There are no toxic gasses and no blood and it is almost completely painless.