We arrest/confine people for many other crimes, not just specifically kidnapping. Similarly people are fined for crimes other than theft. Typically the choice of punishment is not based directly on the actions involved in the crime, except in the case of murder, where the death penalty is used because the state murdering someone is seen as appropriate when that person has been found guilty of murder.
First of all, the state does not "murder" anyone. By definition "murder" is the
unlawful taking of a human life. You may disagree with the death penalty (heck, I do too, but not for the reason you've given), but given the fact that its been approved by the courts and various executive/legislative bodies, it is not "unlawful". Calling a state-run execution "murder" is just cheap rhetoric.
Secondly, it seems like you're still under the misguided notion that its "murder for murder". But its not. Not all murders get put to death. And when a person is put to death, they are typically executed in a way that's more humane than what the murder's victim went through. Yes, the convicted killer "dies" in an execution, but its not exactly a perfect overlap.
Some countries focus on protection of the public, and where possible rehabilitation and making amends rather than punishment.
Those goals are not always contradictory with the death penalty.
An executed individual will be even less likely to be a threat to the public than someone in prison for life. (After all, people
do escape jail, not to mention incidences of jailhouse violence.)
Rehabilitation sounds like a good idea, but given the fact that individuals who are sentenced to death probably wouldn't be released anyways its kind of a moot point.
As for making amends, there's often very little that a convicted killer could do to "make amends". The opportunity to be a contributing member to society is limited from in jail. And its quite possible that the only "value" they could ever provide is the closure that's provided to the friends/families of those they've killed by being executed themselves. (Note: I do recognize that not everyone would have the same attitude towards a "killer"... some victim's families may grant forgiveness. There are no universal rules here.)
Seriously... there is one very good reason to oppose the death penalty: The chance of executing an innocent person. That should really be the only argument anyone ever needs to make. Everything else that people bring up seems counterproductive.
