This is actually more what I would expect, something along those lines.  Not necessarily exactly that, but simply killing Maggie is too easy, too quick.  From revenge, it would probably be more interesting and more manageable to somehow corrupt or taint Maggie.  Ideally, from Nicodemus' POV, that creates a situation where Harry must either kill his own child (thus in Nicodemus' twisted thinking equivalent to him and Dierdre) or live with the knowledge of what she has become.

 Harry having to kill Maggie would be a cruel twisted thing that Nic would love to arrange..  I think he might take it a step further, have Maggie sacrifice herself willingly for Harry as his daughter had done.  If Maggie had to be "put down" because of what she had become, Harry maybe wouldn't forgive himself but as seen in a mercy light learn to live with it.. Sacrifice herself though, perhaps like her mother, I don't think he could.

But wherein Murphy is apparently the worst, surely her ceasing to be Murphy would only be a good thing.

  Really?  If she is no longer who she is at her core being, that is the problem, circumstances are now preventing her from being what she is..  She isn't a good listener,  if that isn't fixed this port in the storm role isn't going to work, she isn't going to be content with that kind of role.

Really?  Mab could literally have ordered Harry to get her the Sword and he'd have been powerless to stop it.  Even without Mab's orders Harry was under the Mantle's influence.  He was so terrified about the Winter Knight Mantle he chose suicide.  If Harry was willing to have himself killed because of the Mantle why would Murphy trust the Sword with him?
 Murphy's judgement was correct that a Sword may be vulnerable under such circumstances.

Not talking about Mab and the Sword...  Oh Murphy was correct that the Sword was vulnerable but then she went ahead and did exactly the thing that made it vulnerable... But that isn't the point,  the point is she is clueless about the post Changes Harry... She wants him to return to pre Changes Harry, that man is dead, she is afraid of the post changes Harry..  Does Murphy even realize that Harry suicided to avoid getting the Winter Knight's mantle?  Or that he is on first name basis with an Archangel?  Well, she might now..

The Sword worked perfectly for her up until Nicodemus surrendered.  She was in the right up until that moment where emotion overran reason.

No,  for the record her ovaries had nothing to do with it....  Love had nothing to do with it..   The Sword is meant to fight Denarians, but if the Denarian surrenders, then he/she is allowed a chance to seek a path to redemption..  Murphy says up front before the fight began, before that anyone knew she had the Sword concealed in that tube, that she didn't believe in redemption for Denarians..   What broke the Sword was her condemnation of Nic, the "damn you.."  Judgement is for God not Knights..

Quite a judgement being made there

Only quoting or paraphrasing not just Michael's words, or Uriel's words, but Murphy's own words... 

No such thing as too many people to love

True, but she isn't indispensable,  Harry has many friends to turn to for love and support is the point..

As a fan why would I prefer her to die? 

If she ceases to be Murphy you might....  If she goes too afar a field from what she originally was..  When the book is perhaps more readable without her than with her... Or it doesn't matter if she appears on the page or not..

As for Murphy not really being meaningful for the stories, that's up to Jim.  Louis Lane didn't have powers but to Superman she was his rock, his port in the storm.  Murphy's meaningfulness might be less about her fighting ability and more about being Harry's port in the storm.

  But in the Superman stories once Superman grew up he and all those around him seemed to freeze in time...  The never aged from Jimmy Olsen to the guy who ran the newspaper, so MS Lane as the perpetual perky inquisitive foil remained plausible.... More or less...  Susan was good in that role until it got serious between them and then seemingly Jim had no
where to go with her, even when she became a half turned vamp, which could have kept her interesting..  While Murphy might remain meaningful as Harry's port in the storm, she isn't his only port anymore..

Jim may have meant it as a joke, but even joking about it says it is on his mind..

I guess it's Michael's time to die as well.  Not every character needs to die when they aren't ready to be on the front lines fighting.  I think what it's showing is just as Harry has outgrown his enemies of the past, he is now getting to a point where his allies of the past just aren't up to fighting the fights he's going to be having...  Marcone, Murphy, the Alpha's, Thomas....  They just aren't on par with Harry anymore, he's too big just as the enemies he's going to be facing are too big.

True, most know perfectly well they are not on par with Harry, but Murphy doesn't...  That is the point, she thinks she knows Harry and what is best for him, but she is in fact clueless about him, this is what made  her a liability in Skin Game to the point of disaster, i.e. the breaking of a Holy Sword..  Someone earlier spoke of setting up what is to come in the next books...  I think we are being prepared for her exit, whether it is a mere break up or something more lasting..  I also think if her role is diminished from being someone who physically fights at Harry's side to some "emotional port in the storm," not just fans in general but her fans will be disappointed..

   I seem to recall a "Tweet" from Jim maybe a year ago give or take that he was  emotional after writing Murphy's funeral...  It created a bit of stir here of course.  The question is, was he really or just pulling our legs? 

I hope you are wrong, because if he goes after Maggie he is dead (I would be so happy if he succeeds, before Harry kills him in a horrible way)

  But that would be merely revenge if Nic gets Maggie..  Very hollow if she ends up with a coin which would be really horrible..

   Police dogs do wear protective vests but it doesn't totally keep them safe..  I think Mouse will, if he hasn't already developed his own form of protection, he doesn't need Harry or anyone else to make it for him.