According to this link the split soul is not a good vs bad soul but rather a snapshot save frozen in time.
Going with this then the difference between the two souls is the added experiences of the Banshee Queen. You can blame some decisions on trauma, being undead and not having as strong positive emotions, and fear of the Maw but not because her “Ranger general good-side” was not present. This is not a Star Wars good and evil Kirk thing this is young Sylvanas horrified by the actions older Sylvanas has done while she was frozen in time right ?
Are you talking of the original work or adaptations like League of Gentlemen ?
Either case does not fit here as the soul shard is not an “other” part of the soul. It is not morally different or less or more repressed. It is basically a backup up of what they were like at a moment of time. The reconnecting of the souls is not bringing anything lost to the original soul. It is simply offering perspective and a chance for reflection.
Jekyll also willingly became Hyde in order to indulge in the hedonism he so desired.
Sylvanas was forced into Hyde unlike Jekyll who was only forced into Hyde when it became clear there would be repercussions for Hyde’s malevolence! Before then he became Hyde of his own free will!
Back to what Uther says though. This is not two different parts of the same person. Both sides of the split soul at the time the soul is split are the same. The only difference occurs after the split in the experiences of the main soul experiences.
This is more of a time travel type of thing where your younger self sees what your older self has become. The Ranger General is horrified by what she has become. The older Banshee Queen is frustrated that her younger self cannot understand the decisions that she has made when at one point they were the same.
As Uther stated Sylvanas’s wound is deeper than his. Frostmourne took more Soul so there was room for Darkness to fester and grow within her. Judging from how her Soul had Red Eyes upon becoming a Banshee it grew very fast!
Uther only had a piece cut out which didn’t influence his Morality while Sylvanas had a big chunk of Soul ripped out allowing her Evil side to fill in the Hole even if there was some goodness in her.
The goodness within her could have also grown to fill her soul. It wasn’t as if he cut out a chunk that represented her morality, her goodness or self control. You can argue that it is understandable or even very likely that anger and hate would grow when you are hurt.
Reacting to pain by wanting to cause more pain and giving in to dark impulses can make one a relatable villain but would not offer any excuse.
Naturally there was a possibility of a hole in a Soul to be filled with either Good or Evil. The Banshee Queen due to her situation replaced what was lost with pure Malevolence which dominated the rest of her.
If she were in a state to fill the hole with Goodness she probably would have been more inclined to do Good Deeds. As it were she could have only filled the hole in her Soul with either Pride, Hatred, Apathy, Rage, Doubt, Despair or Fear!
Fear was the only Emotion that she didn’t use to fill in the hole in her Soul that she could have filled it with.
Alas Death Knight Arthas did not permit her the privilege of filling her Soul with Goodness(not with his constant torture) anymore than Ner’zhul let him have the privilege of replacing his rapidly drained Soul with feelings of Goodness!
Both ended up replacing what they lost with twisted Soul Scar Tissue with Death Knight Arthas removing everything that wasn’t Soul Scar Tissue. Lich King Arthas’s Soul after removing the last fragment of Prince Arthas’s Soul was just a mass of Memories and Sadistic Emotions.
Sylvanas on the other hand never got to remove the old part of her damaged Soul thus causing a constant Moral conflict between the new part(which was malevolence incarnate) and the old(thus explaining why she hesitated to use Kingsmourne on Anduin) which was made worse when Zovaal returned the removed part of Soul which was made witness to a Soul that was Half-Herself(the part not removed) and Half Pure Malevolence(the replacement of that removed).
It kind of reminds me of the Joker in a way. One of if not his main driving points is that we all will be no better than him if we go through enough pain and horror.
The same story that gave us that idea about the Joker also refuted it.
Joker; “…Gordon’s been driven mad. I’ve proved my point… All it takes is one bad day to reduce the sanest man alive to lunacy.”
Batman; “Incidentally, I spoke to Commissioner Gordon before I came in here. He’s fine. Depspite all your sick, vicious little games, he’s as sane as he ever was. So maybe ordinary people don’t always crack. Maybe there isn’t any need to crawl under a rock with all the other slimy things when trouble hits… maybe it was just you, all the time.”
There was a time when I assumed (naively) that that was what they were doing with Delaryn.
Instead of, you know, making a new sympathetic character for THE PATHOS, raising her for no real reason besides petty spite, having her be a brief road bump who was barely noticeable in the Alliance Warfront, be moody at Calia, and then vanish.
There were a lot of odd choices made with that character.
Especially given Sylvanas didn’t raise her after the Burning of Teldrassil. In 8.1 she was like there was something I forgot to do…oh yeah raise the Night Elf version of me.