I respectfully disagree on one point.
I think Kat's backstory was only introduced in this episode in order for her actions to be seen as 'redemption'. I find that rings very false, and a little manipulative.
Kat has been a model officer so far this season, having grown so much from her humble 'nugget' beginnings and subsequent 'wild' period. Had the writers chosen to focus on her existing personal journey, then her sacrifice in this episode would have taken on an entirely different meaning (as opposed to introducing an eleventh-hour 'dark secret' with no real emotional/continuity context).
Bottom line: I believe Kat's sacrifice would have rung just as emotionally true without the clumsy device they used. YMMV.
Excellent episode nonetheless.