Contextually, from God.:D Selecting out of context verses is a poor habit, and leads to narrow-minded thought. There is a time and place for all things, and there is a clear, holistic, body of scripture, mandating discipleship for all peoples. But, in any event...
It is not society's place to seek vengeance, but I would argue that it is appropriate to seek justice-there is a clear difference. There is no scriptural admonition against killing, but, specifically, against murder(e.g., specifically, by definition: the taking of life for pleasure, or profit).
Again, if we are able to make any decision regarding socially appropriate behavior, we are qualified to make every such decision. For a philosophical model to be valid, it has to meet a logical standard, otherwise, my feelings are as much a basis for decision-making as yours, and the process devolves into who's daddy is bigger & who's in control of the guns.
In context of theology, God, as described by Judeo-Christian heritage, makes sense, He is logical and presents himself in a cohesive, understandable manner through His creation. The people were given the Law, and told to follow it; they didn't, and wanted a king. Sooo, we/they were given kings and judges, and nothing has changed that. As a Christian, certainly, the taking of life may, individually, be determined as "wrong", though only on an individual, personal, basis. We are not, however, a uniformly "Christian" society, nor can one Christian mandate the terms of forgiveness for another.
In social context, then, life in prison rewards the violator with 1) the potential of release, and 2) requiring the survivors/victims to pay for the maintenance of the violator- unarguably a condition of slavery to the violator.
Equally, I would ask, what is the place for persons, such as myself, in Christianity, who are capable, by nature, of entirely dispassionate assessment, and/or capable of taking life without either feelings of gratification or remorse(not that I never get mad, but largely, I look at situations for what they are)? If there is a purpose to all things, where does that fit within the Body? Are some, then,congenitally removed from salvation (predestination), or incapable of receiving grace (absence of free-will)? Also, if we accept, as we must, that there is a paradoxical corollary in life after the fall (time to live, time to die, etc., etc..), then we cannot reconcile absolute sanctity of life with direction against child sacrifice, protection of wives/children, etc., with living among those who choose to live in hedonism.
Bear in mind, I'm not saying I'm correct, but I am looking for a cohesive element in a "pure sanctity" model.
Edit: for LongIsland, and others who may not be Ju-Ch adherents, I am uncomfortable with convenient relativism, so I cannot say that all things are equally valid, but I would be in agreement that once an individual is pursing a reprobate mind, as it were, execution is no different than excising cancer- there is no element of evil(i.e., intentioned harm for benefit), it is dispassionate and preservational.