Originally Posted by
Hitei
The problem with this (and it's one of the most common arguments being given by people in this ongoing buzz surrounding the cinematic) is you're making some fairly baseless assumptions on Sylvanas' behalf. She claims that she would never have turned out this way, but the entire point is that Banshee Sylvanas is the same person with less (not gone, mind you as her interactions with Anduin, her sister, etc. have shown) of a conscience. The Banshee didn't do those things because she was mind controlled, or hypnotized, or somehow 'injected' with evil that made her do things she wasn't capable of... she did them because she (that is, Sylvanas the whole, complete being) was always capable of those things to begin with, despite her not wanting to accept that "future" could be hers.
People want to rely on the notion that inner Sylvanas wouldn't have done the same things (and I haven't watched the T&E video, but I assume this is similar to the past/present expressed in that video) but she is the same person who did those things. "Whole Sylvanas" went through trauma (the destruction of Quel'thalas, her death and raising, the subsequent horrors she was forced to do for the Scourge) and it changed her outlook on life slightly and let her inherent selfishness and arrogance develop into a complete disregard for the lives of others or morality in the face of the cycle. She didn't do the bad things because she stopped being whole and was incapable of being a good person, she did them because it was her response to what was done to her.
As a parallel example, Jaina went through something similar with Theramore's destruction. "Pre-Theramore" Jaina was all about peace, and "would never have" demanded the Horde just be put down, or considered flooding the entire city or Orgrimmar, or purged Dalaran of the Blood Elves--but she went through a traumatic experience that altered her perspective on the world and lost her previous values.
If you went into the past somehow, and showed pre-theramore Jaina her Pandaria self about to wipe out the entire city of Orgrimmar, she would no doubt claim that it can't possibly be her, and she would never, in a million years, do something like that. But she is wrong, and it would be rather silly for players to claim that Jaina wouldn't normally be capable of that kind of thing, because we have already seen that she is.
Sylvanas is that. Her "inner" Sylvanas is like a fractured shard of who she was before the trauma, like the time-portal Jaina, unable to accept that she would go down such a dark path as a consequence of what happened to her, but they are the exact same person. It isn't that she could only do those things because she was split, it's that because she was split, we get a glimpse of her 'inexperienced' self's reaction to who she has become. That inner Sylvanas (or a Sylvanas left whole), if she had somehow been raised, and forced to murder her kin, failed to save her people, failed to get justice on Arthas, been terrified of what was waiting for her in the Shadowlands, and been offered a chance to escape fate by destroying everything... would have done the exact same things as Banshee Sylvanas--we know that because they are the same person, the Banshee, the Ranger-General, the Unified; they've just been experiencing different things by virtue of being shattered, with the inner one forced to watch in horror at the person she became.
The purpose of the entire exercise is to act as a sort of magical, metaphorical visualization of what trauma does to an individual. As above, Sylvanas doesn't do what she does because of the split, the split is a sort of artistic representation of how trauma has fractured the person from being good to being bad. The unification is there, like with Uther, as a result (very important) of her finally letting go of her obsession with gaming the system and fear of paying her dues, not as the cause for those things. Narratively, it is also intended to soften (not remove completely) the hostility towards her by showing that some small part of her collective whole still understood that she was garbage and doing evil things, even if she chose not to listen to that sliver of conscience. Much like Arthas' final conversation with Terenas softens him.
The problem is that people are over-applying that. It is not supposed to make you forgive her, and it very explicitly isn't supposed to deflect responsibility. If anything it doubles down on her guilt and responsibility, because she wasn't mind-controlled, or unaware of what she was doing, or even fooling herself. She knew just how wrong all of it was and still went through with it because it was a matter "beyond life and death".