Vengeance
One of the major themes of Six of Crows is vengeance. Revenge and greed are the driving forces of the plot, as essentially every character has someone or something they want vengeance against: Kaz wants to take Pekka Rollins down brick by brick; Inej wants revenge against Heleen, and later against slavers at large; Matthias wants to kill Nina, and Nina wants vengeance for the crimes committed against Grisha; Nina particularly takes vengeance against Jarl Brum, leader of the drüskelle who captured her. As characters succeed or fail on their quests for vengeance, the novel interrogates right and wrong, though no character comes to the conclusion that revenge is not worth pursuing—while Matthias gives up on vengeance against Nina, it's because he decides she isn't guilty, not that revenge is wrong.
Trauma
Trauma—and recovery from trauma—is one of the main themes of the novel. The world of Ketterdam is a traumatizing place, and each of the protagonists bears the scars from their journey to get there. Often the scars are literal. Inej was kidnapped, trafficked, and raped in a pleasure house. When Heleen grabs her, she experiences debilitating PTSD. Kaz was traumatized by being scammed by Pekka Rollins, particularly his experience stuck on a barge full of corpses, then using his brother's dead body as a raft to swim back to Ketterdam; his trauma prevents him from touching people or being in crowds without feeling intensely ill and repulsed. Matthias was wrongfully imprisoned in Hellgate for a year and forced to fight for others' entertainment. Nina, as a Grisha, has been persecuted all her life, kidnapped by drüskelle, shipwrecked, and is now stuck in Ketterdam. Partway through the novel, she sees pyres where fellow Grisha burned alive, as well as cells where her people were experimented on and tortured. Wylan was so severely maltreated by his abusive father that he ran away, and he is horrified by the violence he sees throughout the heist. Even Jesper, lighthearted and risk-loving, is traumatized: He discovers his Fabrikator strength in a fight to save his life, but the discovery is horrible and makes him feel like he loses part of himself rather than gaining.
Nations & Alliances
There are many nations in Six of Crows, and each of the main characters begins with their own national and personal alliances. National ties are especially strong for Nina and Matthias, though they're subtly present for others as well: Jesper is half Zemeni, half Kaelish; Inej misses her Suli family (of Ravkan nationality); Kaz claims to be motivated solely by money, which shows he has fully adopted the Kerch kill-or-be-killed worship of profit. Gang and/or personal alliances layer on top of national alliances. By the end, each of the characters is in some way disenchanted with their previous beliefs. The theme of alliance, particularly to a nation, allows the novel to explore the complexities of the structures the characters initially trusted.
Addiction
Six of Crows explores addiction, particularly its use as leverage to control others. The most obvious example is jurda parem, which multiple groups are attempting to control—it gives a Grisha incredible power, but it causes a debilitating addiction that enables the person who controls jurda parem to control the Grisha who needs it. Additionally, Jesper is a gambling addict with enormous debts; his addiction to risk encourages him to enter violent situations and shootouts, which Kaz uses to his advantage. Kaz Brekker himself is accused of "serving greed" or an addiction to wealth, though whether this accusation is true is arguable. It seems more likely that Kaz is addicted to revenge, and the greed he displays is a symptom of his desire to take everything Pekka Rollins has.
Monsters and Humans
The theme of monsters is explored both literally and metaphorically in the novel. Fjerdans, among many others, consider Grisha to be monsters and persecute or hunt them, even burning them on pyres. Djerholm is full of statues of monsters, and the walls around its center form the shape of a dragon. Matthias calls Kaz demjin, "demon," and many characters call him a monster throughout the text for his "inhuman" intelligence and inclination toward violence. Inej is called "the Wraith," a ghost, and after taking jurda parem, Nina is comforted by Inej saying that someday Nina might be the scary monster story that Fjerdans tell their children. Literal "monstrousness" is juxtaposed with monstrous behavior, like that of Jarl Brum, who takes pleasure in hurting others.
Imprisonment
Like many of the other themes in the novel, imprisonment is explored on both a literal and metaphorical level across the central cast. Bo Yul-Bayur's imprisonment in Djerholm is the crux of the heist, and Matthias is imprisoned in Hellgate at the beginning of the novel. Both Nina and Inej are captured abroad and stuck in Ketterdam. More metaphorically, Jesper is imprisoned by his gambling debts, Kaz has very serious trauma and needs to get revenge on Pekka Rollins, and Wylan has a learning disability, "trapping" him in a state of his father's (and Ketterdam society at large's) disapproval.
Power
Power, and the inequality of power, drives the novel. Between Kaz's influence, Nina's Heartrender abilities, Jesper's marksmanship, and Inej's incredible acrobatics, the protagonists are undeniably powerful. But they operate in a world that undervalues their power and prevents them from moving upward. When Jan Van Eck offers Kaz 30 million kruge, he offers him a small portion of the influence available in Ketterdam; Van Eck's personal wealth is probably far greater. Beyond that, he never intends to give Kaz the money anyway, and he knows that Kaz isn't able to stop him. The heist crew members, while strong, end up being victims in the power struggle between forces far larger than themselves; only in the sequel will they have the potential to dismantle these power structures.
