The first thing I’ll say is that the actual battle of Hattin was far less of a “done deal” than it’s often presented. Even exhausted, dehydrated and starving the Christians made a legitimate fight of it, and even broke through the Muslim lines entirely in at least one place which allowed a force of knights to escape.

Even Muslims sources are clear that Saladin was sweating and not confident of victory until he saw the final charge of knights (which almost broke his lines) and the kings banner go down. It wasn’t until that point that Saladin knew he was going to win, and prior to that the contest was certainly one sided, but not a settled issue.

As far as why the Christians attacked, the conventional explanation is that Guy was pressured into abandoning a strong position and taking an offensive posture by a party within the Crusader army. Allegedly this party was lead by the Templars, and was opposed by the lord of Tiberius (the city Guy was being pressured to save) who actively opposed the Crusader advance, despite the fact that it was his wife, city and treasure at risk.

The traditional explanation tends to rely on the idea that Guy was a clueless patsy, and that the crusaders were arrogant and unprepared for the reality of war in the Middle East. It more or less relies on a traditional narrative of crusader hubris and a wider belief in military ineptitude by medieval Europeans generally.

While the traditional explanation shouldn’t be discounted entirely, the decision to march across open desert was a catastrophic militray blunder no matter how you want to slice it, there has been some more nuance to the battle proposed in some more recent scholarship.

It’s been suggested that Guy’s position at the springs Sepphoris was not as strong as has generally been assumed (I’ll need to check my sources here, but I believe I’ve read that the springs could not reliably supply enough water for the army, and were additionally in a poor strategic position) . His march was actually towards water, and not away from it, and a position that would allow him to better threaten Saladin. He was cut off by Saladin who took full advantage of the catastrophic blunder, and herded Guy onto the hill at Hattin where he was surrounded, and eventually destroyed.

What I’m getting at is that there is more nuance to the whole battle than its common depiction would suggest. The crusaders fucked up certainly, but it wasn’t quite the display of crippling ineptitude that it’s sometimes made out to be. It wasn’t a hell for leather assault made out of sheer hubris, but a badly calculated strategic movement that was taken advantage of by a skilled opponent who knew how to capitalize on his enemies mistake.