Why must the United States Attorney for Massachusetts seek the death penalty against the majority will of the people who were victimized?

from: http://www.bostonglobe.com/metro/2013/0 ... story.html
Apparently, Tsarnaev was ready to plead guilty and be sentenced to life without possibility of parole, and waiving all appeals. The whole thing would be done without the grandstanding and he could have gone into a hole to live out his life, available to BHU types as they study the phenomenon of radicalization of youth to terroristic objectives.
Instead, we'll spend a fortune to pursue his death, against the will of his victims.
This proceeding doesn't serve me as a citizen. Does it serve you?
Some good pieces:
http://cognoscenti.wbur.org/2015/01/05/ ... ue-process
http://cognoscenti.wbur.org/2014/05/09/ ... dershowitz