The Darkling, despite being the villain of Shadow and Bone, works as a well-intentioned extremist with a sympathetic tragic backstory. He has had plenty of upsetting things happen in his long life, from the reason he created the Fold in the first place to losing Alina after thinking he might win her over.
The Darkling refuses to give up on his plans, despite events turning out differently than he expects. The last scene of the series shows him, once again, meddling with ancient powers - which he may not fully understand - to again gain the upper hand for the Grisha and their safety. Ultimately, though tragic things happened in his life, he succeeded as a powerful Grisha soldier who refused to go down without fighting.
His Lover Was Killed In Front Of Him
When he was young, Aleksander's lover Dunia was killed in front of him after the two attempted to escape the king's soldiers to join the other Grisha in hiding. This backstory wasn't in the book, and neither was Dunia as a character; both were added for the show. The Darkling did manage to get away, but Dunia did not. It's clear that this loss is one of the tipping points that causes the Darkling to seek out the magic to create the Fold. Ben Barnes portrays his grief heavily, which makes it the best episode of the season; he is devastated by losing Dunia. This is the first personal loss for the Darkling, and his reaction shows just how unhinged he will become in the future.
He Couldn't Make The King Listen To Him
In his episode 7 backstory, the Darkling fervently attempted to protect the Grisha by appealing to the king of Ravka. Unfortunately, the soldiers wouldn't work with him, and he was forced to resort to ancient unheard-of magic to create the Shadow Fold in the first place.
His plan was that if the Grisha could destroy the Fold he created, years later, maybe they would finally gain respect. Even as a younger man, despite clear respect from the other Grisha and his mother (at the time), the king refused to acknowledge him, and that struck a major chord with the Darkling.
He Truly Believes He's Doing The Right Thing
Towards the end of the season, when it's clear that Alina will have nothing to do with him and sees what he's doing as wrong, it's heartbreaking that the Darkling clearly thinks he's doing the right thing for the Grisha. He did manage to protect them, yes, but they aren't respected. The Grisha are instead feared by the Fjerdans. Just because the Darkling has the powers he has doesn't mean that he should use them to subjugate the world, but he doesn't realize that.
He Loses Alina Romantically
Despite having chemistry with Alina, their relationship is a bit iffy, partly because of the age difference and partly because he is her commander. She's also hung up on Mal. Alina is eventually told the truth by Baghra and fully abandons the Darkling, running into Mal along the way.
It's clear that there is no going back for her. The Darkling realizes this soon enough and it's a crushing blow to him emotionally. "Fine," the Darkling says, roughly, as the start of one of the best quotes of Shadow and Bone. "Make me your villain."
People Turn Against Him
Zoya and the dignitaries all turn against him once they realize what his actual plan is: to expand the Fold. Ultimately, the dignitaries die, but the Darkling does lose his right-hand woman, Zoya, since she betrays him after the attack. Though the Darkling is left for dead, he manages to crawl his way back to life. Presumably believing he can't rely on actual people since the battle on the skiff, he instead resorts to shadow monsters to do his bidding; said monsters can be seen following him in the final scene of episode eight.
His Grisha Aren't Respected By The First Army
Mal and Alina clearly express their derision against the Grisha in the first episode. Even though they are the elite Second Army, the Darkling managing to have elevated their status, they are still not universally respected. After all, Fjerda outright hunts Grisha down as witches. Even after fighting for them for hundreds of years, the Darkling still cannot make everything right for his Grisha.
He Is Outwitted By A Thief
The show gives the audience a fight between Kaz and the Darkling. However, it isn't much of a fight, since Kaz uses a flash bomb to temporarily blind the Darkling before Kaz can be cut to pieces. On the one hand, Kaz gets away safely, but the Darkling doesn't manage to defeat one of his enemies. On the other hand, it is a uniquely badass move by one of the Crows. He was outsmarted by a literal teenager, who has much less experience than he does.
Alina And Mal Escape Him
Alina and Mal also managed to escape the Darkling's clutches, despite the use of the stag amplifier. After the battle of the skiff, he wakes up alone, with no one staying to help him continue his own dastardly quest.
His only successes were in expanding the Fold just a bit more, and in avoiding dying. Given winning Alina over to his plans was one of his major goals this season, that blow has to hurt.
He Can Never Truly Have Everything He Wants
The Darkling, partly due to being the villain, can never really have everything he wants. Even though he has succeeded in extending the Fold by the end of the season, he knows that Alina, Mal, and Zoya are going to come after him to stop him from extending it even further. His mother is also against him, though it is uncertain what her next move is. Someone will always stand in his way. He controls the shadows and Alina the light; they are forever in opposition.
His Mother Disdains Him
Though his mother, Baghra, seems to appreciate what he is doing at first during his backstory, it quickly becomes clear that she thinks forming the Fold is a step too far. He goes ahead with it anyway, and afterward, she stays close to him, but she doesn't trust him. Baghra trains Alina because she was asked to, but along the way she tries her best to make Alina see that the Darkling isn't as good as she thinks he is. Luckily, Baghra succeeds in getting across her own fear and disdain of the Darkling to Alina, which causes her to turn against him.