Good but flawed on one major point

Rating: 6/10 (4 January 2020)

I was blown away with the cinematography, soundtrack, acting of Del Toro and Brolin. And I was very entertained. Great premise, only spoiled by Blunt's character.
Don't get me wrong, I love a strong female lead, and the part was well acted, just not well written. Here's my issue.
Blunt is hired after being a presumably superb firearms/Swat cop after narrowly being shot and taking out a cartel member and discovering a grisly scene of 45 cartel victims secreted in a house, which is booby trapped.
She is then hand picked, interviewed, and accepts the job, knowing they're going for cartel members. Massively dangerous job. That, in fact, is why she accepts.
Why then, does she (and her male colleague) then behave like a naiive primary school teacher?
Why, when on a top secret mission, does she choose to go out, go to a redneck bar and pull some random guy? (who proves to be her would be assassin?) Why does she pull a gun on del Toro and punch Brolin?
I understand the need for some conflict of character between Del Toro at one extreme, and her, but it was like she just didn't get it one bit, even after, about a third of the way in, Brolin and Del Toro literally SPELL IT OUT to her about why she's there. Yet, she carries on with the mission, and still whines about why she's there.
When she was in the tunnel, and her head hit the ceiling, so she takes off the helmet and draws a sidearm when she can hear raging automatic fire? Come on. I know we don't have to be ultra realistic, but from a highly trained firearms officer?
Maybe the director should have shown a character arc. For example, Blunt ends up on the final mission with Del Toro and SHE ends up killing the cartel boss and his family. Now that would really give her and us something to think about. Disappointed, because this could have been amazing with some character progression. Won't be watching it again.