Detlaff did the mass killings because you gave an ultimatum. So the attack logically follows the ultimatum.
The real question is if Detlaff would have given that ultimatum?
Indeed. I talked about the massacre in general because I was a bit lazy to go into detail, but yes, you are totally right.
I 100% buy into the concept that Detlaff would demand Syanna appear and that he would have given some type of threat. And I 100% buy into the concept that if his demand was not met he would carry out his threat. Powerful people and governments do that all the time so it fits.
The real question and issue for me is what would his threat really have been? The writers picked the whole city to enhance the story but I think it would have been something different. A royal person at random till Syanna came to see him or something like that.
That sounds far more reasonable to me, to be honest. I agree that Dettlaff is the type of guy who would fulfill his threats, but I don't think at all that he would threaten to slaughter a whole city just like that.