The sack was preceded by a literal genocide of all Latins/Italians living in Constantinople a few decades earlier. Coincidentally Venetians weren’t really affected since they were already exiled before that. However still it probably made it easier for the members of the fourth crusade to justify the atrocities they committed.

Prior to the massacre of the Latins the relationship between the east and west was pretty good, in fact the Byzantine ruling class was quite heavily westernized and the economy dominated by Italian merchants. The French princess ruling the empire on behalf of her underage son also wasn’t exactly popular amongst the masses. The subsequent coup and massacre pretty much permanently splintered a somewhat productive alliance that allowed Christians to retake much of Asia Minor ane the Levant in the preceding century.