Killing is bad as deemed by society. The societal standard to denote the ugliness of the act is "murder," which signifies premeditation. The awfulness of murder is directly linked to the level-headed, methodical planning of executing a goal to kill.
The purpose of the legal system is to convert values into law with the purpose of ensconcing a value system into a society and culture.
I feel the legalization of hypocritical policy should be avoided, for it debases the value system that decries murder is bad. Legalizing something doesn't make it right; it's merely made legal.
Legalized state killings are murders. In fact, of the two types of murder, i.e., first-degree and second-degree, state murder is the most legally heinous version - first-degree murder, which is done with deliberation.
The ethics that accompany the precept that murder is bad do not disappear b/c a circumstance of murder (state murder) is made legal. It is bad b/c murder is a level of control over individuality that wholly exceeds any other form of oppression.
Capital Punishment is a law based on revenge rather than societal safety; it is the execution of an imprisoned and impotent offender, and, therefore, exceeds government's role to protect and defend.
And if you need more, there is always the argument that there is no coming back from death. A carried out death sentence is final, and, yet, the judicial system is imperfect.
Also, eyewitness unreliability has been well documented. Cases solely based on eyewitness accounts should certainly not lead to death. For examples of the unreliability of eyewitness accounts, go to the below links.
Visual Expert Human Factors: Eyewitness Memory Is Unreliable
Eyes (and Minds) Deceive: Witness Unreliability Casts Doubt on Death Penalty Rulings | Observations, Scientific American Blog Network