It is very important to keep up appearances. If the government murdered people in the same ruthless, or even practical, manner as ordinary murderers, then people might ask the question: What is the difference between the government and ordinary murderers? Could it be that the state is just as evil and capricious as an ordinary armed robber? Because you definitely don't want people even thinking questions like these, official executions must have a very different look and feel than other homicides. The government is supposed to be the "good guys", and so they must adhere to something resembling "good guy behavior". Yet at the same time they have to actually kill a person, which can look bad, no matter how one does it... Sterilizing the needle is just one sort of small (and arguably pointless) act of kindness. Letting the condemned have whatever he or she wants for a last meal, is another.