Use of lethal force as a 'necessary evil'.

A friend posed this question last night with regard to police use of lethal force and I thought it was thought-provoking and could make for an interesting discussion:

"In what other public service profession is it considered a 'necessary evil' to execute a person simply because you assess them to be a threat to yourself, another person, or the public at large?"

Is lethal force the best answer? Having lived in both Canada and the United States, I have noticed a marked difference in the incidence of using lethal force by law enforcement, though Canada by no means has a perfect record. Still, based on my research, police in Canada generally try to deescalate, disarm, and take suspects into custody, rather than discharging their weapons. In the United States, however, too often the response is to shoot first and then it's too late to ask questions because the suspect, or even the innocent civilian, is dead. As we know, this disproportionately impacts people of colour.

So, to the question. In what other public service profession would this be considered acceptable?