There’s no doubt that Tsarnaev committed the crime of murder. Whether he did it for religious reasons, or “under the spell of his older brother” is irrelevant. A person is judged by her or his actions.
And yet here is an opportunity for רחמים — mercy. It is the sign of a strong society to have that capacity, something DAEASH entirely lacks. We cannot let this murderer go free. And to kill him would be to make him a martyr in the eyes of DAESH and those that pervert the Prophet of Peace.
No, the truest punishment would be for him to spend the rest of his life at Pelican Bay, in permanent isolation. No one to gloat over, no one to hear him. No one but himself. Give him a prayer mat, a Koran, and an arrow pointing him towards Mecca. Perhaps in communing with Allah, and his prophet, he might find peace. And still know that he will ever be a forgotten soul, except by those he maimed and the families of those he killed.