Just don’t go and kill them all this time, OK?
The Crusades were, in terms of jus ad bellum, as defensive as the Allied invasion of Normandy. Their practice was sometimes criminal. Their purpose, however, was the liberation of the Holy Land from hostile hands (Seljuq Turks), the securing and restoration of the Holy Places, and the liberation of oppressed Christians.
I am a peace-lover but not a pacifist. As Thomas Merton once said, "I'm not opposed to all war in theory; just in practice."