A lot of tension, suspense, but some bits don't fit

Rating: 6/10 (21 December 2015)

Sicario is tense and suspenseful, to be sure. But some of the bits just don't fit together quite right.
Spoilers: When Kate (Emily Blunt) emerges from the tunnel she sees Alejandro (Benicio Del Toro) pointing a gun at the corrupt Mexican police officer. Kate then assumes that Alejandro must be a bad guy and prepares to fight him. However, she knows the Mexican police are likely corrupt and working for the drug cartel, and she also knows that hours earlier, Alejandro saved her life when he took down the assassin-guy that Emily had picked up at the bar. So it seems to me, Kate would not automatically assume Alejandro was bad when she first sees him at the end of the tunnel. Maybe I'm missing something here, but Kate's reaction in that scene did not make a lot of sense.
Secondly, at the end, Alejandro shows up at Kate's apartment and threatens to kill her if she does not sign the report that absolves Alejandro's actions. She is shaken to her core during this scene. Terrified, she signs the paper, and Alejandro disassembles her weapon before he leaves. He walks out of the apartment, and Kate instantly reassembles her weapon and threatens to shoot Alejandro. What I find so improbable here is that Kate, who is really shaken up from this encounter, can instantly reassemble her weapon. From the way the scene is played, she's in no condition to do this. This is just my opinion, but when someone is really scared, it's extremely hard for her to muster the precise eye-hand motor coordination she would need to instantly reassemble the weapon like she did. Perhaps this is a nit picky point, but it did not feel honest to her character.