Do not seek vengeance but justice
There are rumors on the internet going about that a Syrian Christian militiaman spotted an ISIS warrior, caught him, forced him to dig his own grave and then beheaded the warrior.
If this is real (and I have doubts), then I do not see how it should be celebrated. Even if the guy by chance was founded out to be an ISIS warrior, he should have been detained instead of killed in cold blood. He could have been sent to the proper authorities for judicial process, but I would be careful about glorifying this act. Pacifists will use this as fuel against the just war doctrine. I will not equate him to ISIS, but I still do not think his response was right; Christians are not about vengeance, but love and mercy; the just war doctrine only permits the military or militia to do what is necessary but not beyond that.
“Love your enemies: do good to them that hate you: and pray for them that persecute and calumniate you: . . .” (Matthew 5:44)
These are the words Our Lord Jesus Christ spoke. I am not an absolute pacifist nor do I believe capital punishment is an intrinsic evil, but I don’t support vigilantism. Such punishments should be left to the proper authorities and if said authority is not available, then care must be taken not to resort to excess. Saint Augustine urged that soldiers and public officials “cherish the spirit of the peacemaker” so that they “may lead them back to the advantages of peace” . “Blessed are the peacemakers: for they shall be called children of God”  said Our Lord Jesus Christ. Saint John Paul II wrote that “[t]o desire vengeance in order to do evil to someone who should be punished is illicit” . Vengeance as opposed to justice does not fulfill the role of peacemaker, but rather scandalizes the faith of the culprit and others around them. Those who eliminate the threat to peace are peacemakers, but those who seek vengeance or to go to farther extremes than the enemy are not peacemakers.
 Saint Augustine of Hippo. Letter 189. <http://www.catholiceducation.org/en/controversy/politics-and-the-church/the-just-war.html>.
 Matthew 5:9
 Catechism of the Catholic Church, no. 2302
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