It all might have played out very differently if the Crusaders had paid the Venetians, after assembling in Venice. The other thing that could contribute to channeling the Crusade to its original goals is to butterfly an event that took place over 20 years earlier ---the Massacre of the Latins in Constantinople in 1182. Revenge was a factor as well as greed in the sack of Constantinople.
http://en.wikipedia.org/wiki/Massacre_of_the_Latins
I'm not entirely convinced revenge was a big deal in having it be drawn to Constantinople (in terms of its influence on the crusaders), but it certainly didn't help.
Not really fair (observing this based on the quote by Carroll) to compare the actions of a mob to the actions of an army, though.