When someone does something this vile, we (the people, by the courts) determine that they shall no longer have a right to their life. Whether that be by ending it via capital punishment or by keeping them incarcerated forever, they essentially no longer have a right to their life.
It costs ~$48,000 per year to keep someone incarcerated (at least in MA where I am: http://www.massbar.org/legislative-activities/mba-memo-on-se...)
If a court determines that someone will no longer have a right to their life and their remaining lifespan is, for example, 40 years, we will spend somewhere on the order of $2,000,000 to keep them incarcerated for their lifetime.
Of course, that assumes the value of the dollar is constant (it isn't) and that the price of keeping this murderer in a maximum security prison is the same as the average cost of keeping someone incarcerated. (it isn't)
When we are locking the door and throwing away the key, why not save the $2,000,000 keeping someone alive who we've deemed has no right to life anyway?