From Jerusalem: The Biography by Simon Sebag Montefiore, a description of what the Crusaders of the First Crusade did when they reached Jerusalem in 1099:

The fighting raged there for hours; the Franks went berserk, and killed anyone they encountered in the streets and alleyways. They cut off not only heads but hands and feet, glorying in the spurting fountains of cleansing infidel blood. Although carrying out a massacre in a stormed city was not unprecedented, the sanctimonious pride with which the perpetrators recorded it possibly was. “Wonderful sights were to be seen,” enthused one eyewitness, Raymond of Aguilers, the Count of Toulouse’s chaplain: “Our men cut off the heads of their enemies, others shot them with arrows so that they fell from the towers, others tortured them longer by casting them into the flames. Piles of heads, hands and feet were to be seen on the streets. It was necessary to pick one’s way over the bodies of men and horses.”

Babies were seized from their mothers, their heads dashed against the walls. As the barbarity escalated, “Saracens, Arabs and Ethiopians” — meaning the black Sudanese troops of the Fatimid army — took refuge on the roofs of the Dome of the Rock and al-Aqsa. But, as they fought their way towards the Dome, the knights hacked a path across the crowded esplanade, killing and dicing through human flesh until “in the Temple [of Solomon, as the Crusaders called al-Aqsa], they rode in blood up to their bridles. Indeed it was a just and splendid judgment of God that this place should be filled with the blood of unbelievers”.

The Jews sought refuge in their synagogues, but the Crusaders set them on fire. The Jews were burned alive, almost a climactic burnt offering in Christ’s name. Godfrey of Bouillon took off his sword and with a small entourage circled the city and prayed, before making his way to the Holy Sepulchre.

A ghoulish delight was taken in the dismemberment of the victims, which was treated almost as a sacrament. “Everywhere lay fragments of human bodies, headless bodies and mutilated limbs, strewn in all directions.” There was something even more dreadful in the wild-eyed, gore-spattered Crusaders themselves, “dripping with blood from head to foot, an ominous sight that brought terror to all who met them.” They searched the streets of the bazaars, dragging out more victims to be “slain like sheep”. Each Crusader had been promised possession of any house marked by his “shield and arms”: consequently the pilgrims searched the city most carefully and boldly killed the citizens, culling “wives, children, whole households”, many of them “dashed headlong to the ground” from high windows.

On the 17th [July], the pilgrims (as these slaughterers called themselves) were finally sated with butchery and “refreshed themselves with the rest and food they greatly needed”.

The princes and priests made their way to the Holy Sepulchre where they sang in praise of Christ, clapping joyously and bathing the altar in tears of joy, before parading through the streets to the Temple of the Lord (the Dome of the Rock) and the Temple of Solomon. Those streets were strewn with body parts, decaying in the summer heat.

The Crusaders of the Fourth Crusade, Venetians and Franks, attacked the Christian capital of the Eastern Roman Empire, Constantinople, in April, 1204. Here’s a brief description of what happened from A Short History of Byzantium by John Julius Norwich:

Once the walls were breached the carnage was dreadful. … Not for nothing had the Franks waited so long outside the world’s richest capital. Now that the customary three days’ looting was was allowed them, they fell on it like locusts. Never since the barbarian invasions had Europe witnessed such an orgy of brutality and vandalism; never in history had so much beauty, so much superb craftsmanship, been wantonly destroyed in so short a space of time. Among the witnesses was Nicetas Choniates:

They smashed the holy images and hurled the sacred relics of the Martyrs into places I am ashamed t mention, scattering everywhere the body and blood of the Saviour … As for their profanation of the Great Church, they destroyed the high altar and shared out the pieces among themselves … And they brought horses and mules into the Church, the better to carry off the holy vessels, and the pulpit, and the doors, and the furniture wherever it was to be found; and when some of these beasts slipped and fell, they ran through them with their swords, fouling the Church with their blood and ordure.

A common harlot was enthroned in the Patriarch’s chair, to hurl insults at Jesus Christ; and she sang bawdy songs, and danced immodestly in the holy place … nor was their mercy shown to virtuous matrons, innocent maids or even virgins consecrated to God …

“And these men,” he continues, “carried the Cross on their shoulders, the Cross upon which they had sworn to abstain from the pleasures of the flesh until their holy task was done.”

It was Constantinople’s darkest hour – even darker, perhaps, than that which was to see the city’s final fall to the Ottoman Sultan. But not all its treasures perished. While the Franks abandoned themselves to a frenzy of destruction, the Venetians kept their heads. They too looted – but they did not destroy. They knew beauty when they saw it. All that they could lay their hands on they sent back to Venice – beginning with the four great bronze horses which, from their high platform above the main door of St. Mark’s, were to dominate the Piazza for the next eight centuries. …

The Fourth Crusade … surpassed even its predecessors in faithlessness and duplicity, in brutality and greed. By the sack of Constantinople, Western civilization suffered a loss greater than the sack of Rome in the fifth century or the burning of the library of Alexandria on the seventh – perhaps the most catastrophic loss in all history, Politically, too, the damage was incalculable. Byzantium never recovered any considerable part of its lost dominion. Instead, the Empire was left powerless to defend itself against the Ottoman tide. There are few greater ironies in history than the fact that the fate of Eastern Christendom should have bean sealed by men who fought under the banner of the Cross.

(Hat-tip to Cogito for the Montefiore quotation)