Willingness to kill shows the 'courage of your convictions', in otherwords shows you mean what you say. Domestically this means killing criminals - personally, I can think of worse fates than being executed. Internationally, this means killing the enemy. To preserve ones life, property, and security either at home or abroad requires a willingess to shed blood. If you aren't capable of this, your enemies will rightly see you as weak and attack.
I think one latent assumption in the question is that we separate the nameless, faceless nuking a hundred thousand people, or sending in the marines with big guns and tanks, from the up-close and controlled personal killing of a man by a physician in front of witnesses.