Even if we view Tsarnaev as Satan incarnate, it is wrong to murder a helpless prisoner. It is wrong for governments to take lives without an immensely good and compelling cause.
But, in fact, Tsarnaev is a good example of how complex crime is. Was he a lifelong Jihadist who had spent thirty years involved in the movement? No. He was an angry kid, dragged along by his brother. He got radicalized. He made a horrible mistake, a grotesquely unethical action, and he deserves punishment. At what point does justice have mercy?