Until the closing scene, I thought Jo's ex-husband was the smarmiest  character of the episode, but Mac just couldn't resist the need to whip out  his infuriatingly-patronizing binary morality.
Mac's right.  The  police do need people like Bobby to make those sacrifices in order to  see justice done.  They need good officers like Flack to convince  frightened but decent people to make choices that are counter to their  best interests.  Sometimes, despite all the forensic miracles in the  world, you need an eyewitness who is willing to stand in open court and  point out the criminals.  So, this is not a case of Mac being wrong.   It's a case of him being willfully blind and obnoxiously smug.
As  a Marine and a police officer who has made tremendous sacrifices, he  should understand the weight of the sacrifices the department often asks  innocent people--who never took an oath to serve the department,  mind--to make at great personal cost, and yet, he seems to understand it  only on the most clinical level.  He recognizes that the department  asked Bobby to surrender life as he knew it, and he also realizes that  it carries a great cost to Bobby, but he recognizes this on only the  most detached intellectual level; he does not seem to know or care what  that truly means at gut-level, nor does he exhibit much sympathy for  Ainsley or Bobby's son, who had the decision to sacrifice taken from  them by Bobby's cowardice in refusing to say goodbye, and who suffered  no less intensely for that lack of choice.  For Mac, all that matters is  achievement of the desired end, and the hurt inflicted upon innocent  others to attain justice and his satisfaction is regrettable but  incidental.
To be fair to Mac, Flack doesn't seem to grasp the  magnitude of the sacrifices he asks of witnesses, either, until Ainsley  vents her spleen and her anguish onto his abashed head and he sees  Bobby's son, a son Bobby has never seen because Flack pressured him into  doing the right thing, but at least he grasps it.  He understands that  he and his colleagues are often in the sordid business of manipulating  people's consciences to get the conviction and closure for someone else,  that sometimes, it's dirty and underhanded and painful and unfair, that  perhaps they ask too much in return for precious little.  Mac has no  inkling of this because since justice is all he cares for, he believes  it should be all that matters to anyone else.  He's blinkered by his own  formidable self-righteousness, and it's maddening.
I think Flack  took one look at that little boy and understood the quantum enormity of  what he had asked of Bobby, of what Bobby's decision to do the right  thing had cost him.  Unlike Flack, who got pats on the back and the  satisfaction of bringing down the Foley brothers, Bobby was rewarded for  his bravery with the erasure of his existence, relocation, and no  chance to see his child or his girlfriend again.  Not only that, but  Flack's "asking"(as he noted to Mac, they don't really ask so much as  cudgel them into compliance)assured that a child would grow up without  his father.  All because he had the misfortune to witness a murder.  He  recognized then and there not just the damage to Bobby, but the  staggering collateral damage as well.
Mac never recognizes or  acknowledges this damage, and so, wrapped snugly in his binary worldview  and fixed morality, he spouts platitudes about justice and free will  and blathers witlessly on, deaf to the depth of Flack's internal  struggle.  Justice served is good enough for him, after all, and so it  must be sufficient for Flack as well.
God, but Mac is an  insufferable, self-absorbed cockwaffle, and writing this has inspired  the urge to punch him in the face.
Flack, however, is more human  and decent than he could ever hope to be, and I want to hug him until  the sadness disappears.
Dear Clown  Registry Guy,
You were such a sublime asshole.  I hated you so  much that I think I love you.  You were so awesome that you took away  the bilious nausea inspired by that flat, pointless, placatory DL scene  with EDNA.  Perhaps you could offer them charisma lessons?
Guera
A  decent episode brought down a hair by Mac's proselytizing.
B-