I've never had so many people quote me from this thread, it was weird coming online and seeing so much when this is someone else's timeline. Six alerts and a few other people talking but not quoting. It would take far too long to respond to everyone so I'll just go by points or general themes.
I think many of you guys are thinking about it far too logically and detached. Maybe I wasn't fully clear but these arguments about '200,000 soldiers' or that the Hungarians made it easier but still it was inevitable or that Greeks are supposed to be civilized sort of miss the mark. Don't think about this with our sensibilities as a reader. Think about how an incredibly racist person thinks a different cultural group he thinks is utterly barbaric would respond to this.
D3's mind is good mind you but what I'm bringing up is his, and his alone, motivations and thoughts about the Latins. Don't get me wrong, I agree that any sane person would look at the Rhomans and think they shouldn't mess with them but I'm talking about the motivations of a very different person. This is the man who wrote, "A clever maneuver that cuts off their logistics will not be enough. They will merely make some excuse about ‘Greek perfidy’ as they have always done to cover up their shortcomings, and then resume the cycle." but yet still cut off logistics before the battle. He also said “the Latin at his core only truly respects brute force.” So while Hungary turning traitor is great for the battle it defeats the purpose of "inflict[ing] such a slaughter upon the Latin invaders that they will foul themselves at the mere thought of trying such again. That they will realize that it is not 1196 or 1204 any longer; that we have indeed put on iron. And perhaps, after we have killed enough of them, they will finally learn."
This is why I recommend the incredibly stupid and barbaric thing as executing the noble prisoners en masse. It would shock and disturb the people of Europe to know so many nobles died facing the Rhomans. I am well aware, and Demetrios is as well, that you don't treat the nobles like that and instead ransom them. But Demetrios's goals here seem to be far more violent and poorly grounded, fuelled by particular Rhoman xenophobia towards the Latin West. If he wants to send a message he will do it in blood, as he said. So while yes he probably shouldn't, I think it would be in character for him to do so. It doesn't matter what we think he should do, what matters is what a man like this would do.
Furthermore any argument about Rhoman mobilization is innately moot. D3 paid particular attention to how the Latins treat Muslims, as brave and chivalrous warriors, while the Rhomans are not. See this quote here, from the update and not D3's exact words as the ones above are: "Despite all that has happened, the Latins still think of the Romans as effeminate gold-wearers, not ones dressed in iron ready for battle and for slaughter. The Romans are rich and weak, the ideal victim. They do not care to cross swords with Muslims, for they have a tradition of fear and respect for Muslim blades. But not for Roman. And so they must be taught." It does not matter how many Rhomans are mobilized, they will simply be seen as having won by sheer numbers alone. D3 wants the west to understand that Rhomans are strong warriors, not just a swarm of peasant conscripts that tried to overawe the west with numbers, but instead a people who individually are capable of extreme violence and brutality when the need arises and can do so well. A big army is nothing new, Rhome's always had good organization, but that hasn't stopped the Latins in D3's eyes because they see Rhomans as weak and effeminate gold-lovers. It needs to be clear that these soldiers are mighty men too, and I think that D3 should see the Hungarians turning face as not going to achieve that goal of making Rhoman soldiery appear as particularly skilled and vicious killers in war. They will just appear as perfidious as always to the Latins in D3's mind, ascribing the victory to Rhoman organization, moneying, and diplomacy rather than Rhomans wearing iron and meeting blood with such a slaughter as to terrify the Latin west of even the thought of returning. Because D3 believes, "In the words of Tourmarch Mikrulakes the Latins ‘must be beaten with their own damn tune’. For at the end of the day the Latin only respects brute force."