Though he mellows out pretty much instantly when Saitama assigns the protagonist to be Genos' disciple, implying this is mostly from his desire and desperation to learn from Saitama.
Here, his desire to become Saitama's disciple clashes with the custom protagonist's own, resulting in him becoming very short with the protagonist and even threatening them with violence (and being 'erased') despite knowing they're a fellow hero. Adaptational Jerkass: In the source material, Genos is nice enough to others despite being The Comically Serious.
