The so-called "thief crying 'stop, thief'," means that even though you yourself are a thief, you loudly accuse others of being one.
Another metaphor is that only the officials are allowed to start fires, while the commoners can't even light a lamp; the officials may start fires everywhere, but the common folk can't even light a candle.
The old nun and the bald monk were just like that, these bastards from Dragon Gate had done many evil deeds in the capital, yet at this moment, they were righteously scolding Yusuf Zuck and the others for lawlessly killing people.
On the surface, Yusuf Zuck respectfully bowed to the nun and the monk on the roof, but before he had even straightened his waist, he had already ordered everyone to open fire.
If the other party is an enemy and not a friend, why be courteous? Fight, beat them to death!