Regarding Jaina, her story now is almost 15 years in the making. I believe her character is what her life experiences have made her.
I see it as a case of her finally coming to terms with the understanding of some historically accurate concepts, such as:
1. "History is written by the victors."
2. "Might makes right."
She's had mud (and more) thrown in her face by not only those she thought she could trust in the Alliance as well as the Kirin-Tor, but also by the Horde. She lost Theramore and so much more. She saw how Dalaran's neutrality was violated to accomplish that. She also saw the Horde betray the Alliance at the Battle of the Broken Shore (we, as the audience, know the Horde's side of it, but look at it from the Alliance's Perspective. They don't know that).
She strove for peace all her life and all she got in return was treachery, violence and the death of those close to her.
She's finally understood that the current powers that be won't listen to ideas of peace nor hold hands and sing Kumbaya.
Oh and let's throw in the fact that guilt can be a very powerful motivator. She feels she's guilty of standing aside as her father died while trying to fight exactly what she herself about to start fighting against now.
If you saw the cinematic at the end of Mists of Pandaria, you can literally see the disgust on her face and hear the hatred in her voice when she tells King Varian to "dismantle" the Horde. Source: https://youtu.be/L4V6hk-_30k?t=50s
And then when she thought that Varian was about to carry out her suggestion, you could see the satisfaction on her face (It even shocked Anduin).
While it may be true that the Horde (under Thrall or Vol'jin) may not have been as violent as Jaina thinks it is now (or was under Garrosh), it doesn't even matter now.
There's something that I learned from watching Battlestar Galactica. That is: "Context matters." Don't just look at the end result of an incident, look at how that incident happened and what were the reasons behind it. Context is something that can easily make something look insanely evil or make it look completely justified as a lesser of two evils.
However, in Jaina's case, she's had it up to her head with context and patience. So she's decided to fight fire with fire.
Now that Jaina is Lord Admiral of Kul-Tiras (and by extension) the Alliance navy itself (especially since she quit from the Kirin-Tor), she's will probably carve out the shape of things as she wants them to be, by force if necessary (because, in her view, peace just doesn't work anymore and nobody listens to her so she'll have to do it herself).
A person can only take so much until it drives them to a certain path.
"Hell hath no fury like a woman scorned."
As Harvey Dent once said, "You either die a hero, or live long enough to see yourself become the villain."
Edit: Fixed typos.