It's not that killing is automatically wrong, it's hashing out when it is acceptable and what is not. If some motherfucker is trying to take your life and killing them is the only option you do it or you die. Spare me the morality bullshit. When it comes to a society at large, there are elements within the society that refuse to or cannot operate within it without seriously fucking someone up or killing someone without remorse. They not only devalue human life, but they take it to the extreme of wasting it rather than letting people go about their miserable existence.
Case in point--how many of you here against the death penalty cheered when Bin Laden was killed? What's the difference between that and executing a convicted felon? You're taking a verified factual bad person and eliminating them from society.
IMHO if there is even the slightest possibility that someone could have been wrongfully accused and incarcerated, you don't execute them. If there is 100% verifiable fact that they are guilty and have not been wrongfully acused, and cannot under any circumstances be rehabilitated in any way, shape or form, you either give them the option of exhile to a remote part of the world, like Svalbard, or you off the bastard since they knew the rules and couldn't abide by them. Call it culling the herd if you will.