It was a political gambit by the Papal state and its allies to bolster the religious standing of the Catholic Church in Europe. It was led by opportunist lords seeking wealth and power and fought by religious zealots who were convinced by the clergy that the reconquest of the Holy Land would usher in the Second Coming of Christ. The Catholic Church had been entirely content to watch the Muslim Empire subdue their Eastern Orthodox rivals for hundreds of years, and was only concerned with stopping them when they had political advantage to gain. And so the Papacy declared that anyone who took up arms against unbelievers and heretics in the name of Christ would be vindicated of their sins, and devout followers rallied under the Crusaders' Cross to fight in the Holy Land. Those who didn't have the funds to go stayed at home and killed Jews, along with various Christian fringe-sects. The clergy had no interest in killing these Jews and later forbid these stay-at-home Crusaders from doing so, but their ideology of a Holy War had already taken its toll. Those that had the means to fight in the holy land killed hundreds of thousands of innocent Muslims, Jews, Eastern Orthodox, and Syriacs on their way, even going so far as to exaggerate the number of civilians that had been annihilated. So it wasn't about Christian warriors seeking justice against Jihadist invaders, it was a war conducted for the self interest of warmongers who used religion to manipulate the masses and justify their exploits, and at an enormous cost of innocent lives. Just because the Islamic Empire wasn't innocent doesn't mean the countless Muslims civilians the Crusaders killed weren't innocent either. And of course let's not leave out their often forgotten Christian and Jewish victims as well.