There is no exception because it is not talking about just killings only unjust ones.
Wars, and killings within them, can be just (look up just war theory). The point of a war, at least from a Christian perspective, is not to kill people, but to stop an invasion, protect innocents or the like. If it isn't doing that then the war is not just. If you are wanting to kill, rather than protect innocents or whatever then you are fighting for the wrong reason. This is what is known as the principle of double effect.
The Crusades were intended to stop the Muslim invasion, regain Christian lands, prevent the fall of the Byzantine Empire, allow for safe travel of pilgrims, etc. Those are considered just things to protect. (Obviously not all people in the Crusades had the same motives, but that is on them not the war in general).
Also, just because Christians, Jews, or whoever do something, doesn't mean it is inline with their faith. People justify bad things, ignore teachings, don't actually believe, etc.
To give a more modern example and one relevant to the thread, jumping on a live grenade is not considered suicide because you are not attempting to kill yourself, but to stop others from being blown up. You know you are probably going to die, but since you are not intending to kill yourself it is not an unjust killing. Jesus said "Greater love has no one than this, that someone lay down his life for his friends".