If a sociopath is so bad he must be kept in prison until he dies of old age, at terrific expense to the very population being protected from him, why keep him on Earth at all? 
Prison does that.
The death penalty is revenge, plain and simple revenge.
Not deterrence, and not societies way of cleaning house, although this can be argued as a secondary effect, it is not the real purpose.
It is payback.
Prison means that we as a society would have to house and feed these pos until they died natural deaths. Why should we be penalized forever for their actions? The funds could better be used in helping the surviving families of the people they murdered.
If a sociopath is so bad he must be kept in prison until he dies of old age, at terrific expense to the very population being protected from him, why keep him on Earth at all?