One-Punch Man, Obsessions, and Humanity
There's a pretty strong theme in One-Punch Man regarding people's obsessions and desires. One of the ways normal people turn into Mysterious Beings is by succumbing to their obsessions: Crablante, for example, was so obsessed with eating crabs that he turned into one. Phoenix Man was obsessed with grief and with his role in his cancelled TV show, so much that he merged with his bird costume. Ugmons are a particular type of Mysterious Being whose jealousy and hatred for more attractive people turn them into monsters, such as Fuhrer Ugly.
However, both heroes and monsters have their obsessions that give them strength. For example, Sweet Mask is obsessed with dishing out justice on evil monsters. He kills with such little discretion that he is a borderline monster himself. Turns out he actually is lol. If they're both driven by their obsessions, what makes the difference between a hero and a monster?
Heroes have held onto their humanity, and haven't let their obsession replace it.
Genos is obsessed with becoming stronger so that he can get revenge on the mad cyborg who destroyed his town. He goes to great lengths to achieve this goal, undergoing the change into a cyborg body of his own and often fighting so hard he loses limbs in battle. His obsession is what keeps him going, but it doesn't fully control him yet. Genos realizes Sweet Mask is a reflection of who he could become if he doesn't hold on to his humanity. Before he met Saitama, he was teetering on the edge of losing it and succumbing to his lust for power and revenge - even now he still struggles. Dr. Kuseno warns him to not push himself too hard, and with every upgrade, such as his post-Elder Centipede one, he appears less and less like a human.
Garou is another case. He was obsessed with getting back at society for mistreating underdogs such as himself, so he started training under Bang to be stronger. When he attacked the other students at the dojo, he lost some of his humanity. Over the course of the Human Monster saga, he continues to lose his humanity as he brutalizes hero after hero for the sake of his obsession. However, unlike Genos who struggles to hold on to his humanity and stay a hero, Garou struggles to let go of his humanity and become a monster. He claims he wants to become the ultimate evil, and yet he can't bring himself to harm a child or side with the brainless violence of other monsters. Saitama calls him out on this after their battle, describing his goal of becoming the ultimate monster as "half-assed." This is why even until the very end of the fight, even when Garou gains the body of a giant beast, Saitama just breaks it and refuses to kill him.
In a way, Saitama himself is a Mysterious Being. Eat your heart out, Garou. His obsession with being a hero strong enough to defeat monsters in one punch led him to his ridiculous training, and he actually achieved his goal. However, after achieving this goal Saitama has found he's lost his humanity, or at least a portion of it. The same happened to Sweet Mask with his desire to be beautiful; he tells Saitama that after becoming popular he now feels his humanity slipping.
This is getting long. Basically, ONE and Murata are telling a story about "staying human." There are people in the real world who let their obsessions control them: being rude and callous for the sake of "social justice," neglecting others for the sake of hobbies, neglecting themselves for the sake of work, succumbing to addictions, etc. We need to remember we are human and keep things like love, mercy, and selflessness in our lives as we pursue our goals. Having an obsession can lead you to become more powerful: whether you hold onto your humanity or not is what makes the difference between a hero and a monster.