Because Thomas left before I got there and I didn't decide to burn down the city until I thought over the whole situation and determined that Vale is way more trouble that I initially thought it would be, that's why. Here's the process that led to that:
1. Vale is no longer worth anything to you due to exile, so I bought it for the ducat I had = Fair trade.
2. Getting Thomas, who I need to run the town, to leave with you = Understandable, it's fanofgaming's decision.
3. You collect taxes early = Cheap, but I can tolerate it so things continue to go smoothly.
4. Telling me OOC you weren't going to try to sell it to other nobles, then talk about doing it IC = Deceitful and makes me ill-disposed towards you.
5. Looting the castle = Really? You're really doing that on top of everything else?
6. Finding out the DE incursion is already at Vale's nearest neighbor so it's only a matter of time before they come to town = Get everything I can out of the place and burn it to the ground for the lulz.
I could've stood any one of 3, 4, or 5 with 6 but a combination of two any of those three with 6 makes Vale more trouble then it would be worth to hold it, and all three+6 means that I'll get more out of the city by burning it because at least I have the satisfaction of destroying that massive waste of time and money with my own hands.
@Taricus: Tristan is going to tell her that the DE burned Vale down, and Laythe will never know any differently. You already received a message that they're in the area so you have no reason not to believe him when he says that's the case.