I think some of you guys are way over thinking this.
It’s a very simple story beat.
Sylvanas is just ‘now’ realizing she’s become the one thing she hated the most, she’s become Arthas/Litch King. She’s succeeded in robbing someone of their free will just as it was done to her.
That’s to me what her expression is conveying.
I follow Christie Golden on twitter, I see almost everything she posts, and I saw her post the tweet that Necroxis is referencing here live and in-person.
Necroxis (and others like him) could not be misrepresenting that tweet harder if he tried.
Here is Christie’s tweet:
So pleased that so many of you enjoyed what we’ve brought to you today! For those who are not happy because they think Y will happen, or Z will not happen…I tell you to remember the story is not yet over. There are many many letters in the alphabet. 
Please no spoilers on the feed! Use #WoWSpoilers as many folks weren’t able to tune in today.
This is like Hitler at the 11th hour realizing that killing people is wrong.
Like how did she not see this comings from 10 miles away?
How did Jaina not realize drowning all of the men, women, and children of Orgrimmar was wrong? If anyone knows the futility of violence breeding violence it is Jaina.
Sylvanas is a forcibly resurrected corpse incapable of feeling empathy (or at least, feels it very weakly).
We cannot relate everything in Warcraft back to real life. To do so is folly, missing the point of fantasy escapism, and kind of just downright missing the point of fiction in my opinion.
I think at that point Jaina had pretty much written off the Horde in orgrimmar. Rage is a perfectly acceptable human emotion especially since it happened over such a short period of time. Sylvanas has been doing what she has been doing over many, many, many years against people who never really acted against her. They were just in the way and she tried to take them out.
Jaina was really simplistic in her approach, you kill my people with fire? I kill yours in water.
Some like Genn simply proved to be more difficult to kill and became a constant annoyance to her.
PS. this is why in the justice system something that is premeditated and planned is looked at much more harshly than someone getting so mad that they do something reckless.