Jaime anticipates that the men will try to rape Brienne and suggests she submit to their advances or face being killed, but concedes that if he were in Brienne's position he would make the men kill him.
The men will try to rape Brienne. This is what Jaime anticipates. Jaime suggests she submit to their advances or face being killed. He would make the men kill him. This is what Jaime concedes. He were in Brienne's position.