Tsarnaev deserves to be in a room watching 24x7 videos of Boston finishers of the years since he did his bombing, and at the end of that week watch and wait for 12-24 hours to be executed by a nail-filled pressure-cooker bomb. Shown the futility of his act and then 12-24 hours of suspense, anticipating his own painful death.
Obviously that would not be constitutional, but it's what he deserves.