Yeah but malty is the villain. She’s supposed to do despicable things like that. It drives the plot forward. Hell the series wouldn’t be a third as good without her.
Meanwhile Motoyasu is supposed to be a hero, someone who helps people like naofumi and the general populace and opposes people like malty and the three heroes church. Yet because of a combination of dilution and arrogance he does the exact opposite and continues to do so regardless of the consequences and results until the villains throw him under the bus.
He truly is not worthy of being the spear hero.
A map that has a mountain doesn't have soil in it. You understand this, yes? A representation of something, such as the word "hero", is not that something.
In-universe, Motoyasu is a hero, as a person with high combat ability. He is the person of high combat ability of the spear.
Narratively, he is not a hero, because he is not the principal character of the story. He is a foil. He exists to show the heighten the contrast between Naofumi's character traits and the rest of the characters. This is why he was
also betrayed by Malty and acted differently from Naofumi, this is a method of characterization called characterization through contrast.
Motoyasu is
not supposed to be someone who helps people. Narratively speaking, he is
supposed to fuck things up. Effective character writing is not making every character a flawless paragon of righteousness. Doing that creates a phenomenally boring and predictable story. He screws things up as a method of enhancing the storytelling.