Society is beneficial, and humans have the capacity to appraise the value of society. The role of sending criminals to prison is that we remove them from society for a time, and we hope they come to realize that it is better to behave and then reap the benefits of society than to misbehave and be deprived. That's the theory, anyhow.
The prison time is varied based on how serious the crime is, but some crimes are deemed so serious as to be irredeemable. An individual who would do such a heinous crime obviously does not value society at all, or lacks the capacity to appraise society's value. We remove such elements from society, whereby we give them the death sentence or life imprisonment.
The death penalty does not allow for mistakes, which is why there is a rigorous process to go through until a person is executed. At the same time, these lifetime prisoners are a drain on society to some degree. Others must work to pay for them. For a few prisoners, that drain is not significant, so we would keep them alive, to allow for mistakes.
However, if there were a great number of these prisoners, and the drain on society became significant, we might choose to execute some of them. People such as the B.T.K killer would be the types we would execute.
The death penalty is bad in that it doesn't allow for mistakes, but beyond that it isn't bad, and I disagree that it need be about revenge. There should be no problem with executing heinous criminals who confess, such as the B.T.K killer, unless we have reason to believe their confession is a lie. However, faced with a death penalty, I doubt many would falsely confess, so that point is moot.
There is no good reason to keep them around indefinitely (those who confess and whose crimes meet the criteria for lifetime imprisonment), unless it is for revenge. I hear sometimes that death for certain individuals is 'too good'. This is a sure sign revenge is the motive. Sometimes to not grant the death penalty is to seek revenge.
Who can argue that the calls for the B.T.K killer not to be given magazines in prison is not about revenge?