There are two fundamental reasons why the death penalty should not be imposed in this case. The first is the obverse of the argument that if anyone deserves the death penalty, it is this defendant. That may well be true. But it follows that if this defendant does not deserve the death penalty, then no one does.
In other words, a decision to withhold the death penalty in this case would be a powerful argument against the morality of the death penalty in any case. As a lifelong opponent of capital punishment in all cases, I would argue that not applying it in this case could have a considerable impact on the movement toward abolition.
This abolitionist argument is unlikely to have much impact on the Obama administration, which favors the death penalty, at least in extreme cases, such as this one. There is an argument, however, that could have an impact even on proponents of the death penalty.
Seeking the death penalty against Tsarnaev, and imposing it if he were to be convicted, would turn him into a martyr. His face would appear on recruiting posters for suicide bombers. The countdown toward his execution might well incite other acts of terrorism. Those seeking paradise through martyrdom would see him as a role model.
Whether the death penalty actually deters crime has been long debated. I have little doubt that some criminals may well be deterred by the prospect of execution rather than life imprisonment, though I am not certain how many. But I also have little doubt that some defendants, especially those motivated by ideological extremism, may be incentivized by the prospect of martyrdom.
Source: The Guardian, Alan Dershowitz, April 24, 2013