My main reason for being against the DP is because it is not proven to be a deterrent to murder and other vile crimes against persons. Since it does not prevent those crimes, then life in prison performs the same purpose as the DP....it removes the threat from society.
Otherwise, prison removes them from society and I believe that is the purpose of prison...to protect society. (Supposedly it is to rehabilitate but I see a very poor record there).
After all, prison isn't a deterrent either, the majority of people who go to prison reoffend and go back to prison. I don't hear anyone claiming we ought to get rid of prisons because they're not deterrents.
“The reasonable man adapts himself to the world: the unreasonable one persists to adapt the world to himself.
Therefore all progress depends on the unreasonable man.” ― George Bernard Shaw, Man and Superman