At the moment it looks like she's just really stupid. She knows he's associated with the Lich King and the helmet and co were made on his command. She knows that he employs Kel'thuzad who was the one Arthas invaded Quel'thalas to revive and is the reason for everything that's happened to her up to this point. On the less personal ground she's also seen him ditch Denathrius once he's no longer needed and there's the slight bit about the Jailer operating an eternal torture factory. Nevermind whether her goals are altruistic or selfish, whether she's consistent or not. The idea that she trusts this guy at all is absolutely bonkers.
Now, his motive is still up in the air. My pet theory and what I think is far more likely is that he wants to expand the reach of Death over the other cosmic powers as other powers can circumvent it at present and can grow, while Death is mostly a stable ecosystem. Because that would throw the balance out of whack he got kicked into the Maw and the Arbiter was created, with his family taking up control of the other afterlives or in the case of Revendreth, creating them. We know that Ardenweald at the least existed before the Winter Queen and Maldraxxus may have existed before the Primus. His goal then is just to take back what's his, i.e control of the Shadowlands and then go back to the great game and beat the other powers. What he's told Devos, Sylvanas etc. is all bullshit meant to achieve his aims. He doesn't care at all about the order of the Shadowlands being unjust so much as it represents from him what's taken from him and how those who imprisoned him left themselves open to the machinations of the other expansionistic cosmic powers. Ditto he doesn't care about individual souls.
Sylvanas in that case, could be motivated by either trusting his motive if she's a dumbass, or if the writers are marginally more clever as her meaning to use the souls she punts there into the Maw and his resources to achieve the goal, then ditch him. This would solve a whole lot of characterization problems while keeping her an antagonist, in that she'd consider any soul punted to the Maw as being on loan there, to be freed when she achieves her goals, but who's suffering is a price worth paying if everyone else is free. Hence the spiel she goes on about how no one has any choice anyway - it's an excuse for her to cover both herself and the suffering she's enabled. It doesn't look like they're going this route though, given that she's a mid-way boss in the Gul'dan role. She could survive of course, and Danuser might for the redemption role, but it'd be on a foundation bound to satisfy no one if the inciting cause is her inexplicable trust for someone who's not trustworthy.
Danuser's writing of the character is marginally better than Afrasiabi, but still completely beside the point. The line about there 'still being some of the Ranger-General' in her summarizes it best. No one who's a fan of the character does so because of her role as Ranger-General, everything is based on what she becomes after, that's her inciting incident, her background. It's much like how Arthas without his journey from Stratholme on and his time as a Death Knight is just a generic paladin. He's hearkening to elements of her characterization that are not even blase, but interest no audience. Her detractors hate it because it sets her up to be better, those who like her because it turns her into a dupe who's been faking it for fifteen years and is actually a sad flower child who needed a hug. It satisfies no one except him.Depending how much Danuser wants to piss us off, he might just declare that during BFA the Jailer was "assuming direct control" of her and thus Teldrassil and all that are not her fault. Frankly, by now, I believe there is nothing he won't do to give her a redemption arc.
Again, Illidan nearly destroyed the world on behalf of a demon lord, ditto Kael and Vashj, notwithstanding the whole slavery, drugging orcs business. But his characterization in every separate version hearkened to the appeal of the character and he also only hurt NPC factions no one cares about. And it still was hamhanded in Legion. The entirety of the game for two years was fixated on presenting this character as pure evil who anyone of any persuasion should hate. It was extremely incompetent in some regards on this front, but it still happened and all those things still occurred. And unlike say, Maiev's stint in the Knaak novel, this isn't in a sidebook but was actually in the game for ages, so it can't be easily cut. The proper call would've been to have her usurp the Jailer and become the Big Bad, doing something akin to what she tells Anduin about - you've got a motive, you've got a proper boss fight built up for ages and it gives you more to work with later. This whole angle of making her a dupe for an entirely new character while at the same time reducing her entirely to a victim just doesn't pan out.I agree on the baggage. Danuser seems to want to pretend that BFA and Teldrassil never happened, so he can tell the story of the tragic anti-hero he wants her to be. Unfortunately for him, that just won't work. The players, especially on the Alliance side, do not forget shit like this.
As I said above, it's nearly impossible to explain away a genocide, especially when it is directed at one of the player factions. Illidan did not have that issue and even then I would not call him redeemed. He was a useful weapon that we turned against our enemies, but that was pretty much it. If he had returned to Azeroth we would likely have imprisoned him again.