Undisputed theory could reveal Boyka's real motivation for fight
Yuri Boyka started the undisputed franchise determined to win at all costs, and his motivation may lie in a fight that drives him in the past.

Scott Adkins' Yuri Boyka may have hidden motives for fighting in the Undisputed series. When audiences first meet Boyka in Undisputed 2: The Last Man Standing, he's not only a skilled MMA fighter, but also possesses a unique determination to win. In fact, Boyka considers himself "the most well-rounded fighter in the world." Still, he lost his second fight to George "The Iceman" Chambers (Michael Jay White), showing that Boyka had let his many victories get him. He has to come back to the top in Undisputed 3: Redemption.
Boyka, who showed a marked aggression for the first time in the Undisputed martial arts film series, softened considerably in Undisputed 3, with Boyka beating his opponent with less consumption. Boyka continues to fight Boyka selflessly in her fight for the widowed Alma (Teodora Duhovnikova): Undisputed after he inadvertently kills her husband. The difference in how he fights between Undisputed 2, Undisputed 3, and Boyka: Undisputed suggests a shift in his thinking. The explanation may lie in the largely unexplored origins of the Boyka, involving a phobia of losing a fight developed after suffering a painful trauma.
Boyka's Aggression Is At Its Peak In Undisputed 2

Believing that God chose him to be "the most perfect fighter in the world," Boyka's opponents in the Undisputed Series always bore the brunt of his will to win. Still, the extent to which Boyka went too far in Undisputed 2, beating many of his opponents far beyond the point where they could continue to fight him, shows that there's more than just a highly loyal fighting machine. They illustrate a person who wants to prove something. Also, Boyka's loss to Chambers was a long-term mental blow. This is his only loss in the Undisputed series, which may be a clue that there's more to it than it first appears.
If Boyka has had a major loss in the past, it likely happened when he first started out as a fighter. It would hit him even harder if he made the same mistake he made in the rematch with Michael Jai White's Iceman, underestimating his opponent, only to suffer a devastating loss. The contrast between how brutally Boyka beat his opponents in Undisputed 2 and his more measured aggression in Undisputed 3 and Boyka: Undisputed is such a stark contrast that Boyka's behavior in the former almost seems like a temporary form crazy.
Boyka's killing of two prisoners who drugged Chambers to manipulate his fights may also reveal something about Boyka's state of mind in Undisputed 2 and other series. In Undisputed 2, Boyka is serving time for first-degree murder and is more than ready to kill anyone he thinks would dishonor his record as a fighter. Boyka: Undisputed, even though Boyka is no longer the undisputed villain, he is deeply guilty of his past actions. While Boyka recognizes how much he let his pride get the better of him before, it's implied that a fiasco may have been the inciting event that led Boyka down this path.
Why A Past Defeat Helps Explain Boyka's Motivation

Boyka's perception of himself in Undisputed 2 is closely tied to his in-ring victories, but how he got to this point paints a clearer picture of who he is. Boyka sees himself as "the world's most well-rounded fighter," not just facing one fight at a time, but defending the ultimate title. Boyka started out as a boxer and probably got into the ring because he was passionate about his inevitable winning streak. However, lost one of his early games A particularly humiliating approach likely resulted in Boyka expanding his martial arts style and promising himself that he would never fail again.
Such a vow makes Boyka's drive to defeat Chambers easier to understand. While Boyka's combat skills are superb, the insecurities he may have struggled with in Undisputed 2 could turn him into a different person. Eventually, Boyka had to come back to reality and realize he had gone too far, which he managed to do in a loss to Chambers. With that much-needed humility, Boyka was in the Undisputed 3 and Boyica: Undisputed as the fighter he'd been in, ready to be in the Undisputed 5 as the undisputed franchise leader. True heroes (possibly in Iceman rematch) fight with honor.