I oppose capital punishment in all circumstances, but if there has to be some method for removing from society people who truly can't be reformed, it would be to execute only those who commit murder in prison. 

Unlike in most instances, imposing the death penalty on people who commit murder in the tightly controlled environment of prison might actually have a deterrent effect, because people who commit such murders should know they are likely to be caught and are probably not intoxicated.  Such a policy would also mean that the only people who are executed are the ones who have shown that life imprisonment is not enough to protect society from them.  And I assume that there would be near certainty of guilt, because of close monitoring by prison guards.