The New York Times makes excellent points in an editorial, "There Is No 'Humane' Execution." The editors considered the supposedly more "humane" method of execution, that of using one drug in the case of Kenneth Biros, instead of three, often criticized as painful, in the case of Romell Broom:
This is what passes for progress in the application of the death penalty: Kenneth Biros, a convicted murderer, was put to death in Ohio last week with one drug, instead of the more common three-drug cocktail. It took executioners 30 minutes to find a vein for the needle, compared with the two hours spent hunting for a vein on the last prisoner Ohio tried to kill, Romell Broom. Technicians tried about 18 times to get the needle into Mr. Broom’s arms and legs before they gave up trying to kill him. Mr. Biros was jabbed only a few times in each arm.
...The larger problem, however, is that changing a lethal-injection method is simply an attempt, as Justice Harry Blackmun put it, to “tinker with the machinery of death.” No matter how it is done, for the state to put someone to death is inherently barbaric.
There remains the inevitability that the death penalty will be imposed on the innocent–the ultimate argument against it:
It has also become clear — particularly since DNA evidence has become more common — how unreliable the system is. Since 1973, 139 people have been released from death row because of evidence that they were innocent, according to the Death Penalty Information Center.
An untold number of innocent people have also, quite likely, been put to death. Earlier this year, a fire expert hired by the state of Texas issued a report that cast tremendous doubt on whether a fatal fire — for which Cameron Todd Willingham was executed in 2004 — was arson at all. Until his execution, Mr. Willingham protested his innocence.
So what is the humane answer to state-sponsored killing, otherwise known as the death penalty?
Earlier this year, New Mexico repealed its death penalty, joining 14 other states — and the District of Columbia — that do not allow it. That is the way to eliminate the inevitable problems with executions.