Spoilers about end story of Kul'Tiras Alliance side.
Have none of you ever been so conflicted and lost in your lives that you simply picked a strand of sanity in your vision of the world which helped you not descend into madnes and despair at how fucked up the world is, even if that strand of sanity was not a healthy one?
Have you not seen and understood emotionally what Jaina deals with every day of her life? Abandoning Arthas? Abandoning Daelin? Uther? Failure to protect those she cared about and betrayal from those she trusted?
Her character is being portrayed as vengeful and begun a descent down her current path because her mind half-snapped. She's had no reason to believe in anyone's views of peace anymore and worst,
she's had no reason to believe in herself anymore.
Belief in others through her ideals led her to suffering after suffering. On top of that, she's a woman, which comes without a joke with a certain amount of by default insanity which is unleashed when their feelings are betrayed. Now maybe this doesn't fit Warcraft's narative where everyone is larger than life in a way, but it's a thing to keep in mind.
At any rate, she's been lost, lost for a long time, tired of everyone's bullshit and thankfully she's found a reason to believe again thanks to her mother and her people taking her back again. Unfortunately, her people all have a bone to pick with the horde also, so...it's not necessarily the best environment for her to get her old self back again fully. We will see in time with 8.1.
Believe in Jaina, and she'll believe in the world again.
Edit: removed the last sentence since everyone seems to be hanging on it like I'm trying to start some shit or something.