There's no requirement in libertarian philosophy that requires a libertarian to support keeping or abolishing the death penalty.
I had been under the impression that the whole point of libertarianism is opposition to the power of the state.
Otherwise what is it but an ideology created to defend the lifestyles of those who follow it?
The whole goal, as I understand it, is a maximization of rights and freedoms. It seems to me that a case could be made that those who are known to be murderous individuals are both a danger to others' rights and freedoms and undeserving of their own rights and freedoms, having taken those of other people away, and as such, that removing these people from society helps achieve the goal I stated.
A similar justification could be found for allowing the state to imprison people; indeed, if someone supported neither the death penalty nor imprisonment for the reason you gave, that person would be an anarchist, not a simple libertarian.