The old gargoyle and the missing opportunities
Baghra's "reveal" of Aleksander as a "villain" doesn't make any sense and ctually makes the plot worse. I suppose the intention was: a young protagonist is charmed by an attractive and powerful man, believes that he is a brave revolutionary and a justice fighter with noble goals, and gets entangled in a relationship with him only to find out that his motives are much more sinister and he actually uses her as a pawn, tricking her with talks about "greater good". She feels betrayed, devastated, heartbroken, and disillusioned with the man she thought she knew and runs away to take revenge on him and ultimately stop him. Except it falls apart right from the start. The narrative tries to retroactively gaslight you by including Alina's lamenting about "how long she lived with his lies", but, considering that we still remember what actually happened, it makes her sound delusional or outright dishonest.
The Darkling is not charming. He's not that typical abuser who plays the role of a sociable, likeable, caring, and upstanding guy who secretly beats up unsuspecting women in his dungeon. He has a dark reputation, a dark image, and a whole swarm of semi-mythical gossip surrounding him. And he doesn't seem to mind. That's not how a charming manipulator acts - he doesn't cultivate a socially popular image. Nobody is charmed. Alina isn't charmed either: she initially recoils from his powers and believes he is going to torture her when he simply asks how her day was.
They actually barely interact. They have 5-6 brief conversations, mostly about her powers and amplifiers. Alina herself mentions he's barely present in the palace, and even when he is, he's not in a hurry to meet her. That's not enough to manipulate someone. It's not enough to build trust. It's not even enough to say that you know something about each other.
He interacts with her for political purposes. He doesn't manipulate her to enjoy the power he has over her and her feelings, like an abuser and manipulator does. He doesn't love-bomb Alina or try to convince her that he picked her for her delightful personality. He needs her power; that much is absolutely clear from the start. He doesn't try to humanize himself to her or get her to feel sorry for him; there is no moment of shared vulnerability between them. When they kiss, he seems almost annoyed with the fact he couldn't separate his business from unnecessary feelings. Playing on Alina's insecurities? No, he only encourages her and even defends her against Baghra's attack. Isolation? Also no, she has a full palace of Grisha and non-Grisha to talk to.
Moreover, Alina doesn't act like a victim of manipulation either. The danger of a skillful manipulation is that it's notoriously difficult to get through to the victim, even for their closest friends. The manipulated person is inclined to dismiss any warnings, get defensive, believe that it's just a misunderstanding, find excuses for their manipulator, and get angry at the person who tries to warn them for meddling with their relationship. Alina does none of that. She immediately believes the old abusive hag with no evidence and runs away.
If Aleksander is manipulative, why not continue manipulating her when he captures her after her escape? Baghra didn't provide any evidence; it would be easy to say she is just a demented old liar whose brain was damaged by the wasting sickness. He could also tell her that Baghra abused him as a child to make Alina feel guilty for trusting her over him. All the opportunities were there.
So Baghra's reveal works to the detriment of the plot because:
Alina is basically told that a guy she and everyone else assumed was evil is actually evil. There is no reason to gasp or feel betrayed; the act of a wronged wife who found out her husband is a serial cheater after 20 years of marriage is completely out of place.
The fact that Baghra delivers all the gossip in one go makes Alina's outrage against the Darkling's methods forced and performative. A day ago, Alina was happily swanning around the palace, not questioning Aleksander's goals and strategy once, not even asking questions about him, and as soon as Bahra told her, she suddenly became a staunch opponent of his "tyranny"? Sounds fake. If her discomfort with his actions was a result of her own moral reflection, that would be at least more honest and gave her a more believable conflict.
It robs Alina of any agency, as she doesn't discover information about the Darkling herself, doesn't start noticing disturbing signs and discrepancies in his behavior, and is getting infodumped by a Baghra-shaped deus ex machina instead. Once again, the conflict would be much more meaningful and impactful if Alina actually put some effort into discovering the Darkling's "true nature".
If Baghra truly believed Aleksander was going to manipulate Alina, why wait for months to warn her? It makes more sense to do it straight away, before the alleged manipulative tactics take effect.
Baghra shouldn't have even been there. If Aleksander was truly such a brilliant strategist and manipulator, he should have known that his mother, the only person knowing his Black heretic identity, could betray him. It's not like she ever made it a secret she didn't support him. So he should have made sure his mother didn't meet the Sun Summoner. Instead, he sends Alina to Baghra for training himself. Not very mastermindful of him.
If you think about it, Baghra works as a big bad manipulator much better. She has a more suitable role—that of a wise, grumpy, but well-meaning senior mentor. Who's going to suspect an old lady of plotting devious plans? Besides, Alina has already been brainwashed by a similar old abusive hag - Ana Kuya—so for her, Baghra's attitude seems familiar. Baghra does play on Alina's insecurities: rubbing in her face her humble orphanage upbringing, shaming her for her attraction to the Darkling (the idea that it's unbecoming for a young virtuous lady to lust over a man is probably already hammered into her by Ana Kuya), belittling her for her poor command over her power, and instilling the belief that Alina is stupid, foolish, naive, and can't make any decisions for herself. She also redirects any possible accusations of manipulation onto Aleksander: he is a centuries-old manipulator who knows how to fool naive young girls, apparently, not her. And if shifting the suspicion onto the person you try to discredit is not the most popular manipulative technique, I don't know what is.