Well the Byzantines had localized genocide/extremely violent pogrom targeting all the Latins/Italians living in Constantinople a few decades ago. This combined

Well the Byzantines had localized genocide/extremely violent pogrom targeting all the Latins/Italians living in Constantinople a few decades ago. This combined with various other real or perceived slights made the 4th Crusade a bit more ambiguous. It's not like that justifies the sack but the Byzantines are definitely partially to blame for the animosity that had arisen between the east and west following the first crusade. Especially considering that they together with the pope pretty much started the entire crusade movement (the first crusade was supposed to just be a relief expedition to liberate the recently lost Roman territories in Anatolia).