[Hermann the Cherusker]

To kill a Muslim in a crusade is a sacramental according to St. Bernard of Clairvaux.

[Destro]

http://www.11thcavnam.com/main/shedding_some_light_on_another_p.htm

Photographer Adams later said that he talked to Loan and Loan said "They killed many of my men and many of your people."22 Adams continued referring to the executed Vietcong: "They found out that he was the same guy who killed one of his ---uh---Loan’s officers and wiped out his whole family.23 Adams also expressed regret for what the photograph had done to Loan’s life.

[Destro]

When the Muslim is waging jihad, yes (which is what the Crusade was trying to fight off).