On the subject of prosecuting a "mistake": As a kid, I'd sometimes have this kind of exchange with my mom:
"I didn't mean to!"
"I need you to mean NOT to."
And then I'd get consequences. Not so much for the mistake as such, but for not doing my due diligence and avoiding the mistake.
It makes a lot of sense to me that even though Guyger never intended to kill Jean in his home, and even though doing so was a horrible mistake, she was criminally negligent in allowing circumstances to progress to the point where the mistake was made.
To me, if you are going to carry a gun then it is your responsibility to be pretty damn sure about when and where you use it. In other words, I would put a higher level of due diligence on someone who carries a gun, especially if they do it every day.