The response from younger evangelicals to the Tsarnaev ruling last week was predictable enough. It amounted to what Derek Webb sang many years ago in "My Enemies are Men Like Me": "Peace by way of war is like purity by way of fornication / It's like telling someone murder is wrong / And then showing them by way of execution."
So, once again, we are talking about capital punishment and how (not) to talk about it. The first point that ought to be made is that many of the critiques of capital punishment don't quite get at the best arguments for it. One good friend on Facebook said that killing someone is a bad way of saying we don't approve of killing people. But this response misses the point, just as Webb's characteristically punchy line does. The arguments for capital punishment are not primarily arguments about deterrence or expressing a certain belief that a society affirms. Rather, the argument at its best is that certain violations of the moral law carry with them an appropriate and fitting retributive penalty. Put another way, the issue isn't that 20 people got together and said "we think killing is wrong and since Joe Smith over there killed someone we're going to express our viewpoint by killing him." Nor is it that 20 people got together and said "we think it is socially harmful for people to be killing each other so we will kill anyone who kills in order to deter people from killing." Those may or may not be valid arguments for capital punishment, but they are not the best argument and they certainly aren't the most Christian argument.
What we see instead is that in Genesis 9 as God reaffirms the covenant he made with man in the garden after the flood is that God himself establishes capital punishment as an appropriate response to murder. And note that in Genesis 9 the reasoning is not about deterrence, mere preference, or even some sort of limited cultural norm. Rather, the reasoning is grounding in something innate in the act of murder itself that necessitates a certain response: "From his fellow man I will require a reckoning for the life of man. Whoever sheds the blood of man, by man shall his blood be shed, for God made man in his own image." Indeed, the text tells us exactly what that something innate is--it's the imago Dei, the image of God that human beings possess.
This idea that certain acts demand or provoke certain responses by a necessity that exists outside our control is largely foreign to 21st century westerners, but it has not wholly disappeared from our literature. To take only example, consider the positive example of Lily Potter's love for her son. At the end of Harry Potter and the Philosopher's Stone Dumbledore tells Harry that a love that powerful leaves a mark on a person--not a mark that can be seen, of course, but a mark that still exists in a real and true way nonetheless.
Likewise in The Chronicles of Narnia CS Lewis plays with a similar idea when he explains that the Narnian universe demands a unique penalty be paid by traitors and that, in a wonderful twist, an even deeper magic allows for that penalty to be paid by an innocent third party who willingly submits themselves to it on behalf of the traitor.
In both these stories we see this idea that there is a moral order to creation just as there is a physical order. And just as gravity demands that a rock fall when I drop it, so the natural law demands that a murderer pay for their crime with their life.
None of this, of course, is to say that capital punishment as it currently is practiced in the United States meets the standards of justice in scripture. There is strong reason to believe that capital punishment in the USA has a very strong racial component to it. What's more, in the states most prone to using capital punishment there is a particularly high danger of them executing an innocent man in their rush to justice. So we must distinguish between arguments that capital punishment is innately unjust from arguments that the American application of capital punishment is unjust. You can deny the former while agreeing quite strongly with the latter.
That said, in this sense a case like Tsarnaev's can offer much in the way of clarity. There isn't a question of Tsarnaev's innocence in this case. Nor is there a question about his race. In this case we know he is guilty and we know there isn't some sort of racially motivated injustice in play. So in such a case can we justly execute a person? The testament of Genesis 9, which is structured in such a way that it can't simply be dismissed as a mere proof text, says that we can.