You`re wrong, crusaders initially didn`t plan to sack Constantinople, but to make Alexius, son of Isaac, emperor and to overthrow Alexius III. Alexius promised to return his empire to Roman church, to give money to crusaders, to take part in their Crusade and then to keep some of his soldiers in Holy land. When they did it and Alexius and his father became emperors they refused to pay the crusaders and even started to fight them. So the crusaders said that they will take that he promised befor in any case. Both emperors were overthrown by Murtzuphlus and that Murtzuphlus decided to wage war against cruseders, but he was defeated and Constantinople was taken a second time. Moreover there were 3 fires in Constantinople, two of them - during the siege, and the last - because of quarrel between Venetians and Francs and their battles on the streets. So the main reason of disasters is this fires. And they were not planned by crusaders.
Nevertheless some of crusaders came to Syria (the plan was to come to Syria, then to defeat Egypt and then to return Jerusalem), but they were defeated because their forces were too small. And the general forces of crusaders (they never rejected the plan to fight against muslims) were defeated by Bulgarian king and they weren`t able to wage war with new enemies.
Phil is correct...the crusaders failed to make up the fees required to pay the Venetians as alot of crusaders travelled to the southern ports of italy and the city of genoa to reach the holy land...Also you left out the capture of Zara