The Angry One wrote...
Again, risking war without consulting the Triumverate is stupid.
And if the Arishok didn't consider the bas of Kirkwall a threat, then he is stupid. His position in the keep was indefensible. If Hawke hadn't removed him, then Meredith's forces would have, or the circle.
And in the infinitely low possibility that the Arishok's limited forces held out against the guard, the Templars and the mages, the Chantry response would've flattened him.
The Arishok is a third of the Triumverate, so to claim that he has no power whatsoever to take such an action is stupid. He may need to consult the Triumverate, or he may not. We do not know the inner workings of the Triumverate system.
The whole of the Qunari are of the opinion that the Bas are incapable of standing up against the might of the Qun. Sten of Origins believes this. He thinks that the Warden would be capable of felling a great number of Qunari in his/her effort, but he still believes the Warden would fail.
Even disregarding that, if the Arishok doesn't consider the Bas of Kirkwall a threat he is rather smart. Can you name a Bas in Kirkwall (save for Hawke) who actually was a threat to the whole of the Qunari? Meredith's forces, nor the Circle mages, had done no significant damage in their efforts to stop the Qunari. They may have done so in the future, but their efforts at the time are do not reflect that assessment. It is hard to take the claim that they would have performed better in the future, without proof, seriously.
It should also be noted that the Grand Cleric was not taken to the Keep. It could be that the Templars were protecting her very well. It could be that the Arishok is smarter than you give him credit, and he knows that killing the Grand Cleric would bring a Chantry response sooner than just taking the city while leaving her(and the Chantry) alive and unharmed. Since we don't ever get to look over near the Chantry, we cannot see the situation. So either option is possible.