Well after hours on hours thinking and watching theories, I think it really just that he recognized the maws power and was shocked by Anduin who also shares or (well now shared) exterior characteristics as blonde hair, golden blue armor, king etc.
I very much htink either Sylvanas feels empathy for him since he basically pretty much goes through what she has been through right after she became a banshee or she realizes that Arthas wasnt completely himself either. Dont get me wrong, he wasnt mind controlled when Silvermoon and she fell, it was his call, but at least the order to resurrect KT wasnt completely his, just like it wasnt her decision to turn Anduin against his will really.
Also it wouldnt fit to Arthas last words…“Father, is it …over?”. I think if we are to see Arthas he wont be the LK version.
And I dislike it because Sylv/Arthas arc got me invested in the lore and made me start playing but all seems to be part of a much greater matter.
Just look at the map (the titans made) of the cosmic forces, and watch closely on it around the titans and around Azeroth/reality. Those runes that are used to imprison things are all around it. Then think about Sylvanas words “This world is a prison”. There is so much more going on than what we know… Im so curious to where this all leads us …