God gave the command to stone people in the OT. Stoning was not an immediate death, but took some time usually. This seems to prove that we are not always obligated to dispatch the guilty as quickly as possible or with the least amount of pain.

Thus we see in some Christian kingdoms some criminals were killed by decapitation or hanging, but some especially heinous cases were killed by drawing and quartering, impalement, or other slower and more painful means to stress the evilness of their deeds. These kings and executioners were not necessarily evil men who inflicted these cruel deaths and these deaths, too, were examples of just justice despite their cruelty.

And if we are allowed to kill convicted terrorists for their crimes, and to kill them in dreadful ways and not always in the most painless manner possible, we are allowed to inflict some lesser pains upon the guilty in order to save the lives of our soldiers and civilians.