First of all, I would say that when we evaluate historic events, we MUST distance ourselves from the 21st century and put these events into their true historical context. We can't pass judgment on them from our perspective - we have to understand the setting in which these events took place.
The Crusades took place in the middle ages, which was a pretty barbaric time, no matter your religious persuasion.
Islam was on the move, and Islamic tribes had been invading Europe. One of medieval Islam's tenets was one world religion - Mohammedism - and domination of that world. In less than 200 years from the death of Mohammed, they had overtaken Palestine, Syria, Mesopotamia, Egypt, North Africa, and the South of Spain. The Moslems even crossed the Pyrenees, threatening to stable their horses in St Peter's at Rome, but were at last defeated in 732, just one hundred years from the death of Mohammed.
By the 1100's, they had conquered and dominated Persia, Afghanistan, and a large part of India and in the twelfth century they had already become the absolute masters of all Western Asia, Spain, North Africa and portions of Italy. In the thirteenth century, Muslims overthrew the Byzantine Empire. From Constantinople, they even threatened the German Empire - but finally were successfully defeated and pushed back from the Danube in 1683. So - from about 900 AD to nearly the EIGHTEENTH century, Islam threatened to overrun all of Europe.
Some people would claim that they are now succeeding in that venture - but that's a topic for another thread.
We can't truly understand the Crusades without realizing that the whole of Europe was threatened by the spread of Islam. The Crusades were as much a DEFENSIVE movement as they were religious.
This was an age of battlefields, of massive armies on foot and horseback, of invaders and princes popes and kingdoms.
When the Roman Empire fell apart, the governing of Europe fell into disarray. Kingdoms were up for grabs. The only unifying force throughout Europe was the Church. This was the era in which the Church's power and reach expanded immensely.
As the Church grew in influence, Europe began to stabilize, and the Middle East was swallowed by Islam, the issue of the Holy Land began to grow. Remember that in this era, most faiths, including but not limited to Christianity, placed great importance on holy places, relics, etc. Since about the third century AD, Christians had been taking pilgrimages to the Holy Land - probably earlier than that, but we have records from that era.
In 1009, Hakem, the Caliphe of Egypt, ordered that the Christian holy places in Palestine be destroyed. Though Jews, Christians, and Muslims had until then lived pretty peacefully side by side in the Holy Land, things took a definite turn for the worse.
The Crusades were not simply European armies intent on overtaking Palestine. These groups also consisted of men and women who were simply devout Christians intent on making a pilgrimage to the Holy Land while they were still able to do so.
People joined the Crusades for a wide variety of reasons. Certainly some did so for selfish gain. Others did so for adventure. Others had little choice - they were ruled by kings and princes who demanded their allegience.
It's easy to judge from a distance. But it's also common for people to leave out the HUGE piece of the puzzle that is the threatening spread of Islam during this time. Eastern Christianity especially was under intense attack.
The Crusades were a Holy War in many ways - and not simply an offensive one. The Crusades were a direct response to the aggression of radical Islam.
This does not excuse the barbaric actions of individuals, or the greed of popes and kings.
As a Christian, I vew this era much as I view the Reformation, or the American Civil War. There were many reasons why people picked up the sword - some were noble, some were led by their conscience, some were selfish, some were devout, some were simply adventurous.
I am sure that the Muslims who were fighting to spread their faith across Europe and the world fought for many of the very same reasons.