To re-cap your opinion, this girl made a terrible, terrible decision at age 15, and no amount of reflection, guilt, and rehabilitation will ever make her suitable to re-enter society, even a half century down the line.
---at this point I would like to mention that we are already at a disagreement, but just to entertain your viewpoint, we will act like I agree with the above statement----
Ok, so she is in prison, where she poses no threat whatsoever to anybody on the outside. She costs a lot of money to take care of. A lifetime means a whole lot of food, energy, water, and man-hours at the prison. We should kill her.
EXCEPT, it costs much much more to kill somebody than to keep them alive. There is more to pay for than the bullets for the guns, electricity for the chair, rope for the noose, or the lethal injection. You have to pay for multiple appeals trials, lawyers and other legal fees.
So basically, your argument is totally invalid.