I will admit that I am not as "balanced" as you in my view of the world, but even if we set that aside, the Fourth Crusade wasn't just "an attack on co-religionists". Countless works of art from antiquity that had been saved from destruction elsewhere were totally annihilated, whether as the result of direct looting and destruction or in the fires that the Crusaders set. Thankfully, the Venetians tried to save what they could, but the other Crusaders melted down precious gold artwork for its base value, destroyed statues just for the fun of it and burned books. It certainly ranks in the top ten acts of barbarism, along with burning the Library of Alexandria (question mark on who is responsible for that), the destruction of Baghdad by Hulegu Khan and the modern-day Bamiyan Buddha destruction. It was a loss not just for the Byzantine Empire, but for humanity as well.