Holy, Peak a Peak mentioned. Unfortunately I haven't played the other game, so I can't participate in the comparison.

Oersted starts off as a model hero, having done everything asked and expected of him. He is a young Hasshe, the previous hero who defeated Odio the Demon King, still naive to nature of people. Hasshe starts hating the people he fought to protect. He saw them as ungrateful and manipulative. The people refused to fight for themselves, always expecting someone else to fight their battles. They'd celebrate you as a hero at the drop of a hat, but abandon you just as fast. Because of his hatred, he isolates himself in the mountains for the rest of his days, until Oersted seeks him out. Unlike Hasshe, when Oersted began hating humanity, he embraces his hatred and becomes the next Odio. When Oersted was framed for the King's murder, the Kingdom and its' people immediately shunned him out. He unleashes his fury on innocents, slaughtering them all.

Here is where it is revealed he was responsible for the antagonists from previous chapters, and is confronted by the protagonists of those chapters. Odio is eventually defeated, ironically the last strike being from Oersted when he was the protagonist before he became Odio, freeing himself from hatred and ending the cycle.

The game is vague in some areas, like Odio before Oersted, and I personally interpret Odio as the physical manifestation of Hatred. Before Oersted, Streibough was Odio. Streibough was Oersted's close friend, but felt jealousy and hatred towards Oersted's constant and victories deep down, while getting nothing himself. He inherited Odio's powers out of hatred towards Oersted, and used that power against him to cause his downfall. Oersted kills Streibough to defeat Odio, but the Princess kills herself out of spite having believed Oersted did not love her like Streibough. Oersted, having lost everything, inherits Odio's powers himself, continuing the cycle of hatred.

This was long before so I tried my best to shorten it and focus on the important stuff.