And I know that people have reasons for what they do - just as Tsarnaev had his own reasons for getting mixed up in terrorism.
In the pro-violence camp, pro-revenge camp, your reason for wanting an execution is clear: simple vengeance. I have yet to hear a moral argument in favor of this. People with morals they are proud of usually do not openly advocate for vengeance. It is widely understood to be an empty and destructive force.
Of course I understand the desire for vengeance. It is based on our ape biology. Humans have a built in desire for reciprocity. When a stranger offers you some help, we feel a need to repay him, or else we feel guilt and fear taking something for nothing. In the same way, when someone cheats us or insults us, we feel bubbling up within us a rage which demands we get back at them.
The evolutionary reason for this reciprocity is clear - it is a proto-morality, an animalian morality, which was the first stirring of the construction of a society with us. Without this first reciprocity in our species' past, we would have struggled to build an ultra social society of strangers. Reciprocity can be seen in diplomacy even, where the general rule is, everyone starts out friendly but cautious, and when someone helps them, they generally help them back; when someone turns on them, you turn on them too and try to make them regret it. This is how reciprocity can help regulate behavior.
So how does that fit into the death penalty? Reciprocal logic - ape logic - says, a man who spills the blood of man shall have his own blood spilled by man.
The only way this can be justified is as a deterrent. Yet we know now in 2015 that the death penalty is not an effective deterrent - especially not in the case of terrorism (most of us know that our efforts at violent retaliation have only bred more terrorists).
This is the dark side of reciprocity, the bitter, barbaric downward spiral of violence that we see in Israel and a host of other violent places, including the USA.
So if we know it is not an effective deterrent but still want it, what is our moral justification? Revenge for its own sake. And how do you justify that? Anyone who has ever exacted their revenge knows that mere seconds after you finally finish the job, you feel dirty and empty. The catharsis is short lived. You look at your friend's bloody nose and crying face, or in this case the lifeless body on the table, and feel nothing. Certainly you feel deep down that no wrong was righted.
So what's the point then?
Do you all sanction revenge for its own sake? How can you?