Who are those who apply the lethal injection?

My question is who are the people who carry out the executions in the USA (and elsewhere) Are they correction officers? volunteer prisoners? Are they aware, that justifiable or not the execution, they become murderers? It is a very subjective matter but I would like to have some feedback. A public executioner even legitimate is unethical and still he is a killer. After someone has completed the execution of the condemned, goes for breakfast(usually executions are carried out late night or very early morning) like nothing happened? In his mind he things that he did right? I just try to get inside the mindset of an executioner and his conscience. Thank you.