This argument is always an emotional minefield that oftentimes carries the baggage of a whole host of tangential arguments. So I will limit my response to two very specific conditions.
#1) Guilt of the person is 100% without doubt
#2) Victim/Victims are innocent and blameless
Going from that premise, imo the death penalty is almost always justified.
But what about the argument that life imprisonment is the harsher punishment? This is EXACTLY why life imprisonment should NOT be the punishment. A punishment should fit the crime and be just. The reason I favor the death penalty has nothing to do with my vindictive streak of making evil doers suffer. No. My reasoning for the death penalty is because it is the most appropriate and deserving punishment to fit the crime.
Life imprisonment is cruel and inhumane because as the years pass we change. Are we the same person at age 45 as we were when 19? Are we the same person at 67 as we were at 25? No. In effect, life imprisonment punishes someone for a crime someone else committed while living in their body X years ago.
However, the death penalty (if administered promptly) punishes the person who committed the crime and is justice. Not vengeance, not revenge, simple justice.
I feel as a society we have hypocritical views of life and death. Truth is, we all directly or indirectly make decisions that result in the life and death of our fellow man everyday. When we walk pass the homeless person and refuse to give them a dollar, buy blood diamonds, buy drugs, buy products from offshore sweatshops, vote for inept politicians who craft our foreign policy... However, we justify those actions, we rationalize away the blood on all of our hands so we can take these lofty idealistic positions about not killing those who deserve to die.
The only respect I have concerning anti death penalty positions are from true pacifists. Because at least a pacifist is logically consistent. Whereas most anti-death penalty people are not logically consistent and support other types of killing (self-defense, war, etc).
For you anti-death penalty people, remember your position next time you change the channel because you are uncomfortable when a "you-can-save-this-child-for-33-cents-per-day" commercial comes on. Because that is a life and death decision you just made through your action and inaction. Truth is, we make life and death decisions every day, so own it. And in the case of the Boston Bomber, he deserves the death penalty for no other reason than it is the most appropriate punishment to fit the crime and it is just.