One of a number of things. The oath was broken at different times in the minds of different people and they were all camels back increments.
Exactly, the Fourth Crusade epitomises the excesses of the Crusades. But if you actually look at it, and the events behind it, you will find that the vast majority of the Crusaders did not set out to conquer areas of Croatia or establish the Latin Empire. It was shortages in funds (and a pitiful turnout Crusaders), the influence of the Venetians (who had a feud with the new ruler of the Byzantine Empire), and the lobbying of Alexios IV (the son of the deposed emperor and a potential powerful ally in further Crusader campaigns), that forced them to redirect to Zara and Constantinople. For this they were excommunicated and widely condemned throughout the Christian world.
Pretty much everything about the Crusades was popularised by Runciman's history of the crusades in the 50s but doesn't hold up too well
Ugh, I hate that guy.
He's not the only one. He was just doing it after everyone else was supposed to have stopped and doing it really well.
It'd be fun to blame Runciman or Sir Walter Scott, but really you have decades of deliberate misinformation, presenting the history that they believed would create the world they wanted rather than what facts supported. And following on centuries of propaganda and etc.
And all that followed itself by lazy arrogance, patronising assumptions that the accepted narrative must be upheld as 'the people' aren't smart enough to grasp reality and just rote-based repetition of misinformation generation after generation.
The majority of school textbooks still report before Columbus Europeans thought the world was flat, despite no actual historian, or anyone with a university degree in history, ever believing that ever in the whole history of history.
So given the whole culture of simplified and falsified 'easy' narratives it's not really fair to pick out one of the perpetrators as the villain.
Although ignoring all that, the idea that Scott is responsible for 9/11 does make me happy.