Spectral Glider
Member
It's not any form of a mistake. It's a huge one that goes against Mike's characterization as a cool, collected and experienced. I'm struggling to think how even a rookie would leave his captive in plain sight on the road if he wants to draw as little of attention possible. Especially so if the intent of the writers was to show the innocent's death spurring Mike to take down Hector. If that was such a big concern then Mike just didn't give enough thought to that part of his plan when we've known him to be the opposite of that.
If Mike's plan in the finale wasn't affected by that civilian's death then his decision is more egregious because he's purposefully stoking the fire and shaking the bee's nest. Killing Hector would have gotten him and his precious grand-daughter in deep shit. Mike out to know better than insist on taking on the Cartel because they threatened his family. The coast was already clear and he was no longer a concern for the Cartel.
Mike got his ass killed because he hitched his wagon to Walt even though he knew he was a ticking time bomb. Mike is not this inhuman, infallible super antihero that you think he his. The writers have gone out of their way to humanize and explain Mike's motivations going back to seasons 4 and 5 of BB and even further expanded on him in season 1 of Saul.