With regards to Fumito, did he really love Saya despite ruining her life by brainwashing her, hiring people to play as her "friends" and having these people killed in a gruesome way? Or is he only obsessed with her blood and the Elder Bairns that he's willing to turn innocent people into them and eventually became one himself?
Yuka. She goes along with Fumito's plan where people got killed and yet, she eventually became the governor of Tokyo where she abolished the Youth Ordinance Bill which was implemented by Fumito in the first place. It's unclear whether she joins Fumito just to get what she wants and yet, she doesn't have any qualms for all the terrible things that he did. In the stage play, it's suggested that she's an Opportunistic Bastard.
The Watchers. The SIRRUT gang imply that they're the Secret Police enforcing the curfew and dragging away all who disobey. When we actually see them, they just stand around and tell people, not even impolitely, to go home, and they only really get hostile against Saya, who's been labeled a dangerous criminal. It's entirely possible that they're just standard police officers telling folks to move along, and not affiliated with Fumito's private army. Saya even fights them non lethally.
There are also some who want to see a revenge-driven story but they ended up disappointed as most of scenes show more of the SIRRUT hackers.
Darkness-Induced Audience Apathy: In line with the anime series, the movie may let you root for Saya as she pursues her revenge against Fumito. But the ending just drops it off where it is revealed that Fumito is in love with her and that his actions are basically his attempt to ensure her suvival and to restore her ability to feed on humans which he failed. Regardless, none of these can redeem him for the countless crimes that he had committed such as having the rest of the civilians in town massacred and turning Mana's dad into an Elder Bairn. At the end, Saya goes on an aimless journey. (see Esoteric Happy Ending below)
Draco in Leather Pants: Despite that he tormented Saya in the first place, Fumito had a lot of admirers due to his Affably Evil personality. When it’s revealed that his actions are actually his attempts to cure Saya’s dependence on blood, it resulted to tons of fanarts of him being sweet and lovey-dovey to Saya. Never mind that his behavior is more of a stalker Bastard Boyfriend who trapped his object of affection in his own Wish Fulfillment fantasy.
Esoteric Happy Ending: Saya gets her revenge but what's the worth of it when she found out that the man who tormented her emotionally and psychologically, killed many people in the TV series and turned Mana's dad into an Elder Bairn happens to be obsessively in love with her and all that he did is for her own survival and to fulfill her desire to feed on humans which he already failed? And considering that one of the people who helped her get to Fumito happens to be in league with him all along, Saya would probably experience more trust issues and never touch a coffee mug again. She can't even go back to Mana after she found out that she killed her dad at the beginning of the movie. Her Walking the Earth at the end of the movie just shows that Saya would end up alone because she might not ever trust people again and end up hurting them if she does.
There's also some Fridge Horror in this: since Fumito failed on all of his experiments and was unable to restore Saya's ability to feed on humans, how on earth will Saya survive in the future? Considering that the Elder Bairns are dying out and Fumito's experiments to turn humans into Elder Bairns had failed, Saya doesn't have enough food supply for her to feed. And given that Saya disrupted several of Fumito's experiments in the past (though the experiment in the TV series was interrupted by a third party), she unwittingly delayed Fumito's chance to discover a cure for her blood dependence. This also makes her revenge on Fumito meaningless because not only she killed the only person who could keep her alive but she destroys any means of her survival which lead to her imminent doom.
Fanon: Viewers speculated that Saya not returning to Mana and friends after killing Fumito is her price for sword that she got from Watanuki earlier.
Foe Yay: Saya and Fumito which was started in TV series.
Iron Woobie: Saya is still recovering from her traumatic experience of being brainwashed in the TV series and having nightmares of her "friends". Though this is not stop her to get revenge on Fumito until she found out he's in love with her and his actions, regardless how atrocious they are, are his attempts to ensure her survival.
Les Yay: Saya and Mana. Of course, CLAMP would do that.
Misaimed Fandom: There are loads of fanarts of Fumito x Saya after he revealed that he's in love with Saya and his actions such as his experiment in the Ukishima province and turning humans into Elder Bairns was for her survival and his attempts to cure her inability to feed on humans with the implication that he turned into one when he failed. Of course, fans willingly ignored that he killed thousands of people, turned Mana's dad into an Elder Bairn and most importantly, emotionally and mentally abused Saya.
Narm: Some of Saya's lines in the Spanish dub, like "I'm looking for a sword... that makes me INVENCIBLE!" This is particularly egregious compared with the same country dub of Blood-C, which was highly praised.
Watanuki only appears to provide Saya her sword, but he manages to bring some of xxxHOLiC's awesomeness with him.
Yuka's scenes, firstly as Fumito's associate and later as Governor of Tokyo, also stand out.
Rescued from the Scrappy Heap: Saya. After her incompetence and airheadedness in the TV series (though that was Fumito's fault), she proves herself to the viewers that she's definitely badass.
So Okay, It's Average: While reviewers praised the movie to the animation (with all that funding from the Japanese government put into good use) and the dark atmosphere, they still find the story and characters mediocre and boring. But they acknowledged that it’s way better than the TV series since it has a different director.
Surprisingly Improved Sequel: Although The Last Dark underperformed at box office, it managed to Win Back the Crowd after the mixed-to-negative reception of the TV series, to the point many a critic actually lamented that the average viewer would still need to watch the anime in order to enjoy the movie.
They Wasted a Perfectly Good Character: Asides from Mana and Kuroto, the rest of the SIRRUT members don't have much background which makes you doubt about being resistance fighters against an organization with a private army and the capability of creating and controlling human-eating monsters. This goes the same with Yuka who only appears in two scenes and yet, it would have been interesting on what kind of role she contributed to Fumito's organization and possibly, her interaction with Saya if only these two ever meet again. The 2015 play, Blood-C: The Last Mind expanded her character, making her more of cold-hearted bitch.
The movie may have become more interesting if they show more background of Fumito and Saya, particularly his motives and his obsession with her, how they both first met before the events of the TV series, and of course her origins and/or adventures before meeting him. The 2015 stage play seemed to answer these questions.
Also, some people would have liked to see Saya's reaction after meeting again with Yuka, her supposed friend, after the latter's apparent Heel–Face Turn.