I believe I can answer your question - it's because Dunccan has a sense of pride, honour and integrity. Rather than roll over and let our city be crushed under the proverbial Parrian boot-heel, he chose to take decisive, if ultimately unsuccessful act
If words like 'pride', 'honour' and 'integrity' are somewhat alien to you - well, that's a reflection on your character, isn't it, and we can all judge you on that basis.
I fully expect a tirade of abuse for that post - well, if you want to waste your time doing that, then it's your problem - It's just that i have a little respect and faith in my Field Marshall.
Written by my hand on the 8th of Hindyear, in the year 1038.