Odin has never been like the other gods
From the start Odin was unlike the other gods: he showed no interest in destroying humanity, staying silent, observing and acting only at critical moments.
What makes Odin suspicious is that he seems to know more than he shows. In Record of Ragnarok, Odin keeps acting in secret behind the scenes, and the appearance of characters tied to Norse myth along with secrets about the past suggest this god may be pursuing a bigger plan than humanity’s survival.
Odin’s plan may involve reviving something
The most notable theory about Odin in Record of Ragnarok is that he is trying to restore an ancient power. Later the manga reveals a big secret about the past: before the current gods there were Primordial Gods with power far beyond theirs, and Odin appears to be trying to bring these beings back. Each battle does more than decide humanity’s fate: the deaths, shifts in the balance of power and chaos among the gods may be creating the conditions Odin needs. If this is true, Odin does not need the gods to win Ragnarok — he only needs Ragnarok to happen.
Does Odin need Round 13 for his final plan, or just to eliminate someone who knows his secret?
The appearance of Sakata Kintoki in Round 12 makes this fight different. Kintoki represents humanity, but also reveals that he knows about Odin’s plan to revive the Primordial Gods.
So this fight is not simply Gods vs. Humanity. It is also a confrontation between Odin and someone directly obstructing his scheme. That could explain why Odin must step into the arena himself: he needs the win to level the score for the gods, but also has to remove a threat who knows too much. If Odin defeats Kintoki and carries Ragnarok to Round 13, what happens next?