I see all of your points, but I have to say I'm against the death penalty. I have many reasons for this, but I'll tell you the main one.
Many convicted people on death row have been later found innocent and exonerated due to newly found evidence, or after discovering prosecutorial misconduct or whatever. These are innocent people that all of us, as members of this club called the United States, who allow the death penalty, would have murdered. If we haven't done this already, which we almost certainly have, we will, at some point, know for a fact that we murdered an innocent person. At that point, we are all murders. And we, in turn, deserve to die.
That's the paradox of the death penalty. Lock the murderers up forever, definitely. But if we kill people that we're "pretty sure" killed someone else, even if the evidence seems terribly conclusive and emotions run high, it remains an incredibly dangerous legal environment. And if you don't think it happens, it does:
http://www.aclu.org/capital-punishment/innocent-north-carolina-man-exonerated-after-14-years-death-row