You should read the vows taken by the Medieval Knights. They promised to protect the helpless, protect & defend the Church, and all of them had a small Relic in the hilt of their swords. That said, the medieval times were even more violent than today is. When they spoke of “peace” in England & France, for instance, they were referring to a condition in which the whole country was not at war (as in a rebellion against a king, which involved everyone). To them, “peace” involved limited conflicts with neighbors, personal enemies, etc… Those who were in the “upper” classes were trained to be knightly warriors from an early age, and began serious training, usually at age seven, by serving an adult knight who would train them. They were sent from their home to the home (castle) of a friend or ally of their fathers, and trained until they were considered proficient enough to become knights, usually at age 17 or 18. They actually enjoyed fighting, and when there was no local conflict, they took part in formal combats, like a fair, but could often be permanently disabled or even killed in such “fun”. To them, in that age, it just added some “spice” to their “sport”!
Also, the conflicts and killings which took place in the name of the Church was a part of the culture of that time. Thankfully, as the Church grew and matured, such events were stopped. However, at that time and during that age, it was not considered a sin, or evil, but a normal part of their lives. Read some historical fiction – there are some very good writers who cover these things, and explain them from the point of view of those who lived in those times. Since God certainly understands the thoughts of all men, He certainly understood them. Even the Israelites wiped out some peoples “in the name of God”, and their prophets of the time not only encouraged them, but told them it was “God’s will”. Perhaps it was then, but God has been slowly teaching us to be more mature and not to kill in the name of religion, and that war must be a “justifiable” conflict or we should not go to war.