Why is a bullet in the head considered inhumane but injecting somebody with lethal chemicals is not? Both of them end you pretty damn fast; the bullet in the head being cheaper.
Also, the Actus Reus of murder is "the unlawful killing of a human being in the Queen's peace". Since a state execution of a convicted criminal using the death penalty is considered a lawful killing (examples include self-defence, soldiers/police not using excessive force, and of course the death penalty), the Actus Reus of murder is not fulfilled meaning no murder took place in the eyes of the law. Since you can't have a Mens Rea without an Actus Reus, the issue of "malice aforethought" cannot be used to prove that it is murder.