Second, it certainly was and is appropriate for Christians to defend themselves and the innocent and helpless against attacks, which is exactly what the crusaders were doing. There were certainly abuses during the crusades, but an immoral action during a war does not detract from the justice of the cause of war. The immoral action should be condemned, but the war itself remains just
As for the Sack of Jerusalem, here is a summery taken from Henri Daniel-Ropps:
In the summer of 1096, various contingents of Crusaders began making their way to the Holy Land. There was no overall leader, so knights from different areas followed their own overlords. As the crusade progressed, the dominant figures were Bohemond, leader of the Normans of Italy; Godfrey de Boullion, leading the contingent from Lorraine and the Low Countries; and Raymond of Toulouse, with knights from southern France. The contingent under Raymond was the largest.
The main crusader contingents arrived in Constantinople by April 1097, and in June took nearby Nicea from the Moslems. A week later they began marching east, through arid wastelands and a blistering sun. The local Moslem commander thought the knights could be easily conquered, the more so as they had divided their forces into two columns. On July 1 he attacked at Dorylaeum.
Bohemond and his men bore the first onslaught. He exhorted his men to stand firm and sent messengers for help. The Normans held until Raymond’s men arrived and then a contingent led by Bishop Adhemar. The Turks fled, having suffered five times as many casualties as the Christians.
The crusaders reached Antioch in October. The siege of the city was long and difficult. Some of the less dedicated leaders weakened and returned home. But Raymond, Godfrey, and Bohemond held firm leading charges and resisting enemy attacks. Finally Bohemond, with the help of a traitor inside Antioch, broke into the city and opened the gates to the rest of the crusading army. Antioch fell to the Christians, but they soon found themselves besieged by a Moslem relief army. Conditions looked grim, but on June 28, Bohemond called forth the entire Christian army. After a final hand-to-hand struggle, the Moslem army was routed, and Antioch was secure.
Finally on January 13, 1099 Raymond led the Crusaders on the final march to Jerusalem. They won a series of fairly easy victories and on June 7 arrived within sight of Jerusalem for the first time, viewing it from a mountain, which pilgrims had long before named Mountjoy.
But the siege of Jerusalem was even more difficult than the siege of Antioch. The sun shone pitilessly and the wind from the desert drained the moisture from the flesh. The Moslems has poisoned the wells near the city, and men would lick dew from the grass or dig into the ground to find moist earth. But then one of the priests with the crusaders saw a vision of Bishop Adhemer [who had died the previous November] who asked the army to fast and then walk barefoot around the walls of Jerusalem begging God’s help. If they would do so, the victory would be theirs.
The men had loved the Bishop and responded to his request. The Crusaders had renewed confidence and courage, and on July 15 the final assault was launched. Godfrey led it, from a wooden siege tower, at one point even holding up a cracked beam with his own back. His men flung open the Gate of St. Stephen. Through it came the Normans and then the main force under Raymond. Jerusalem was taken.
As the men entered the city, all their pent-up frustration erupted and they went wild, looting the city and killing many innocent people. This behavior was totally against the promises they made at knighthood, and marred what would otherwise have been a splendid victory. Neither Godfrey nor Raymond, however, participated in or approved of the sack. Most of those who participated in the sack were excommunicated.