I did two years of Crusade Studies from the age of 16 to 18. I also wrote my Honours dissertation on the Crusades (primarily reasons why it was launched)
Right, here is my take on it.
There are several theories put forth, some more believable than the others;
This one doesn't carry too much weight as it cost a proverbial arm and a leg to go on crusade and most nobles that went had to borrow money to be able to afford to go.
2) French Proto Imperialism
This interpretation of events comes from historians of the 19th century who, living in an age of colonialism, settled on this being the main drive behind the crusades (without ever trying to prove it).
3) Channeling violence out of Europe
The average knight was trained from an early age to hurt, maim, and kill. The only problem was that they were doing it to each other. The crusades presented an opportunity to put all that training and violence to a better use.
4) Faith and religious zeal
People that went on crusade really did believe that by doing so they would earn "points" that would enable them to atone for sins and get into heaven. The Peasants crusade and the The Childrens crusade are good examples of this kind of belief.
5) To reclaim the Holy Lands
Of course this was part of it but you also need to consider that the timing doesnt exactly match up. Pope Urban launched the first crusade in 1095. Jerusalem had not been under Christian control for more than 400 years when the first crusade was launched.
6) Unification of the Western and Eastern Church
Pope Urban wanted to be the man that rejoined the two churches, and he saw the crusades as the best way to do that.
The 6th idea has been growing in popularity of late and I tend to side with that and the 4th reason. Urban was an articulate and savvy pope. His call to Crusade apparently had people tearing their clothes into strips so that they could take the cross there and then. The average person, the people with new idea of any paple machinations, went on crusade because of faith. Going on crusade was about as horrific an undertaking as can ever be imagined and nobody would subject themselves to that without some kind of higher motivation.
If you want to look at first hand or earlier written accounts of the crusade, you need to check where the source material comes from. The three we looked at most in school were the Gesta Francorum (written by a crusader, the Alexiad (written by Anna Comnena, and the writings of Ibn al Athir (a contemporary of Saladin's). The Gesta Francorum gives great insight into the day to day runnings of the crusade from the position of someone not too high up. The Alexiad should be taken with a big pinch of salt. Anna Comnena, the daughter of Emperor Alexius hated the crusaders and her view is decidedly one sided. I don't remember too much about Ibn Al Athir to be honest.
As for the atrocities, they were commited by both sides. The Crusader however were particularly brutal. This brutality must be looked at within the context of when it tool place. You are dealing with people who are thousands of miles from home, starving, sick, scared, outnumbered, and never knowing what the next day held. The brutality they exercised upon taking Jerusalem was just that, brutal. But it was also the release of months or years of fear, anger, and frustration which had built up on the journey from the west to the east. But as I said, the crusaders, the knights and retainers, had one purpose in life, to fight. Expecting them to not to have been brutal is just unrealistic. Just take a look at the pogroms they launched against the Jews of Europe while on their way to the East. Just look at what they did to Constantinople during the 4th Crusade. You can't expect well trained, violent, motivated, faith-filled warrior to have done anything less.