Andy,
My argument is completely logical. The questioner is making the point that in this case an innocent person would have died if we'd had the death penalty, and suggesting that's the main reason we shouldn't have it - ie because innocent people could die.
My response was to state that just because an innocent might die is not sufficient reason to rule something out, giving armed police as an example.
You've now introduced a moral element to the argument by saying it is unbecoming of a civilised society, to sit in a court and plan a killing, rather than doing it in the heat of the moment with the intention of protecting others, and that's a different question really.