This is exactly what you pointed out - not keeping your promises and engaging in wanton butchery made future battles more difficult and yet there were men who did it anyway. Godfrey didn't do this, and neither did Saladin, and that meant they were a cut above the other men of that time. And that's not my 21st century opinion either, William of Tyre writing in the 12th century thought so too. There was a fundamental moral difference between Godfrey and his most of his fellow First Crusaders. _That_ is the point I'm trying to make, so when you say there was no fundamental moral difference, you're incorrect.
I have no interest in de-legitimizing any group apart from the far right. I do so by pointing out that they in 21st century, knowing all that I've pointed out, chose a battle cry used by butchers in the 12th century. I'm not Christian, I'm not Muslim, I'm not even remotely religious. I'm sorry you think I have an agenda against Christians in general, but that's just not true.
I think it would help discourse if you didn't immediately assume the worst possible intent when you see a comment by someone. Please consider doing that.
Crusaders did not have any easily accessible slave markets to sell their captives to, since unlike Muslims they were not practicing mass open slavery at the time, they were far from home, and the captured people were very numerous. So they killed them. This was their most self interested choice possible.
The Muslims had access to liquid, nearby slave markets, and the captives were fewer, so they sold them. This was the most sold interested outcome possible. To do anything else would be to throw away war profits. To break a promise of clemency in exchange for surrender would be to make future battles more difficult.
To try to set this up as some fundamental moral difference is deeply disingenuous and indicates some deeper level of motivated reasoning intended to de-legitimize one group in particular.