^^^ Actually, the apparent painlessness of lethal injection is hotly debated, at least the version carried out in parts of the US. It's argued that it simply makes things easier on the people ordering, carrying out, and viewing the execution (and for voters who support it) to give them peace of mind, rather than being a more "humane" form of execution for the criminal, if you'll excuse the contradiction of terms.
Opponents of lethal injection believe that it is not actually humane as practiced in the United States.
Opponents argue that the thiopental is an ultra-short acting barbiturate that may wear off (anesthesia awareness) and lead to consciousness and an excruciatingly painful death wherein the inmate is unable to express his pain because he has been rendered paralyzed by the paralytic agent.
Opponents point to the fact that sodium thiopental is typically used as an induction agent and not used in the maintenance phase of surgery because of its short acting nature. Following the administration of thiopental, pancuronium bromide is given. Opponents argue that pancuronium bromide not only dilutes the thiopental, but (since the inmate is paralyzed) also prevents the inmate from expressing pain. Additional concerns have been raised over whether inmates are administered an appropriate level of thiopental owing to the rapid redistribution of the drug out of the brain to other parts of the body.
Additionally, opponents argue that the method of administration is also flawed. They state that since the personnel administering the lethal injection lack expertise in anesthesia the risk of failing to induce unconsciousness is greatly increased. Also, they argue that the dose of sodium thiopental must be customized to each individual patient, not restricted to a set protocol. Finally, the remote administration results in an increased risk that insufficient amounts of the lethal injection drugs enter the bloodstream.
In total, opponents argue that the effect of dilution or improper administration of thiopental is that the inmate dies an agonizing death through suffocation due to the paralytic effects of pancuronium bromide and the intense burning sensation caused by potassium chloride.
Opponents of lethal injection as currently practiced argue that the procedure employed is designed to create the appearance of serenity and a humane death, rather than actually providing a humane death. More specifically, opponents object to the use of Pancuronium bromide, arguing that its use in lethal injection serves no useful purpose since the inmate is physically restrained. Therefore the default function of pancuronium bromide would be to suppress the autonomic nervous system, specifically to stop breathing.
http://en.wikipedia.org/wiki/Lethal_injection