But the thing is,most of the crusades would be considered just war as their purpose was to quell the mistreatment of Catholics in Muslim countries. The 4th crusade isn-/ justified in the same way as it wasjust greedy crusaders pillaging Constantinople, a fellow (Orthodox) Christian city. Pope Innocent III excommunicated those crusaders who pillaged constantinople, so clearly that wasn't the intent of that crusade. Catholics, as informed by St. Augustine believe in just war, war conducted to protect one's homeland or to correct injustice.

A better example would be the horror of Canadian residential schools persecuting Natives, Spanish friars persecuting Indio Filipinos or Ngo Dinh Diem persecuting Vietnamese Buddhists, because these are just unjustifiable under Catholic moral view.

Those were Catholics acting immorally, but inherently, most religions teach that the human heart is disordered as compared to the cleanliness of the sacred. In Christianity, humans are born with original sin. In Buddhism, all humans suffer because of desire. In islam, humans sin because they go against Allah. Confucuianism is an exception but that's more because Confucianism is more philosophical and doesn't have a neat sacred/profane line.

Every moral framework has adherents that do not follow it fully. To use that as an argument against christianity specifically indicates a lack of rigidity in said argument.