Why Does Cersei Think Bron Will Kill Her Brothers?

Based on everything we know about him and his fond experiences with both Tyrion and Jaime, it makes zero sense for him to even think twice about killing either men.

He's rescued both men on a number of occasions, and although he refused to stick his neck out for Tyrion when it came to fighting The Mountain, that as merely common-sense speaking, since going up against The Mountain is certain suicide.

Is Cersei really that dumb as to think Bron, of all people, would willingly agree to kill her brothers?

I really don't know how to take this plot development, because either way it's stupid. If he chooses to betray Cersei they'll be a collective 'duh!' on the part of the audience in response to the most obvious and failed attempt at tension in the show's history, and if he does choose to kill either Tyrion or Jaime, it will be completely out-of-character, and the show will have definitely jumped the shark, or whatever the GOT equivalent is (froze the dragon?)