In Mexico, Harper double-crosses both Banks and the FBI, first shooting Banks to stop him from committing the assassination and handcuffing him to the balcony, but then killing the thugs who hired him and taking their key for the payoff from the hit. She tells Banks that she expects the FBI will be so happy to catch and question him that she'll depart with the payoff without much problem, which is why she leaves him alive.
In Mexico, Harper outwits both Banks and the FBI, initially shooting Banks to prevent him from carrying out the assassination and binding him to the balcony, but then eliminating the thugs who hired him and taking their payment key. She informs Banks that she anticipates the FBI will be so eager to apprehend and interrogate him that she can leave with the payoff without much issue, which is why she spares his life.