Recently, when the US state of Georgia decided to execute Troy Davis, and the state of Texas decided to execute Lawrence Brewer, many bloggers wrote some heartfelt and moving articles arguing against the death penalty. I wrote one myself outlining what I think are the reasons for abolishing the death penalty.
Many of the Christian blog posts on the subject focussed on the commandments of God and/or Jesus as a basis for the ethical argument for abolishing the death penalty.
The trouble is, there will be many Christians who think the opposite, and will probably find some Biblical text or other to justify their position.
I think you probably can make quite a good case that Jesus was against the death penalty, as in the story where he saves the woman taken in adultery from being stoned to death by saying "Let he who is without sin cast the first stone". On the other hand, he is also recorded as saying "I come not to bring peace, but a sword" and "if thy right hand offend thee, cut it off"; so in order to work out what Jesus' ethical stance was on anything in particular, we have to select the texts that support our argument. So wouldn't it be easier to work out whether something is right or wrong without reference to Jesus' views, or indeed God's commandments, which are similarly ambivalent ("Thou shalt not kill" as part of the Ten Commandments, but lots of injunctions to stone people to death for various infringements of the Law).
The death penalty is not wrong because Jesus was against it, or because it's against God's commands as handed down in the Bible. It's wrong for far more basic reasons than that; because you can never know to what extent the person was responsible for what they were doing when they committed murder, or to what extent they might change. They may well be innocent, as evidence is not 100% reliable. Killing is wrong because it cuts short someone's life and does them extreme harm.
The Golden Rule (attested to by every religion) says that we should do unto others as we would have them to do unto us - and being killed is pretty high on the list of things we would not like to have done to us. (The fact that a version of this rule has been worked out by every major religion suggests that it transcends cultural context and is based on universal human experience).
God's commands (and Jesus' ethical stance) can be interpreted one way by one group of people, and another by a different group of people; so the Bible is not a reliable guide to ethics. It's got some rattling good stories in it, which when pondered can produce some interesting insights, but I would not use it as a guide to ethics.
So what is the source of morality? According to Richard Holloway, author of the excellent book Godless morality, morality is based on weighing two conflicting good things. So, in deciding whether abortion is ethical, one weighs the good of the life of the foetus against the good of the mother who may or may not bring it to term. In the case of the death penalty, it cannot benefit the victim of the crime to have the perpetrator killed. Society should be protected from the possibility that the perpetrator might repeat their crime, but the life of the perpetrator outweighs the cost of maintaining them in prison as opposed to killing them. There is also the very valid point that carrying out the sentence places a burden of distress on the people who carry it out.
The source of moraliy is not some absolute command handed down from on high (the very absolutism of which can often cause more distress than it alleviates) but the pragmatic considerations of the context in which the ethical decisions must be made: who benefits? who suffers? and to what extent?