One of these days I’ll have to stop using “Yugoslavia,” but for now it still seems to me to be the most efficient term to use to refer to the northern part of the Balkan peninsula. Sorry for any confusion.
Your question is a great one and is very natural to ask. There’s a distinction to be made between Crusades against people who were not Christians (such as the Slavs) and Crusades against people who claimed to be Christians but had been judged heretics. In the latter case at least, “evangelism” in the form of the more benign activities of Inquisitors was tried first, and only if the heretics refused to change their ways was coercion applied via the temporal authority. The thinking was that heresy was analogous to a cancer that would spread if it were not quickly eradicated, and the Church hierarchy believed it was saving many souls (over which it had a paternal responsibility in a spiritual sense) by preventing doctrinal error from spreading.
Whatever the targeted people, in nearly all cases there are energetic temporal powers (such as the king of France) eager to use a Crusade as a pretext to extend their reach, and that was a factor not present in Christianity’s earlier centuries. As I mentioned in the lecture on the nobility, Crusades were used by the clergy as an outlet for the violent impulses of the warrior class; to their thinking, the nobles were going to fight someone, and better for it to be a non-Christian or heretic than an orthodox Christian.
Christian kings and nobles, with no New Testament model on which to pattern themselves, usually tried to emulate Old Testament figures such as David, who was often at war with Israel’s enemies.
Hopefully this helps give a little insight into the thinking behind some the Crusades on the “domestic” front.