The following contains spoilers for One-Punch Man by One and Yusuke Murata available in English via Viz Media.
One-Punch Man has pit Saitama against Garo on multiple occasions, and they've never failed to provide the readers with quality entertainment. However, Garo and Saitama are more than just rivals. Garo is Saitama's villainous and lesser counterpart, and his experiences and transformations are good indicators of how the bald protagonist could stray from the right path.
Their long-lasting feud finally ended in Chapter 166, wherein Saitama planted Garo's face from outer space down the Earth with a punch. Garo then realized the unsurpassable gap between them, succumbing to self-pity. He mulled over his failures and sulked about failing to achieve his life-long dream of becoming evil supreme. While he believed that no one could empathize with him, he realized his myopia upon realizing Saitama was still holding Genos' core.
Garo's origin started with him setting his sights on a goal -- becoming the embodiment of fear and evil. He was nothing more than the above-average villain who sought to become stronger. It was just that Garo's preferred training method was through battles with his life on the line, and his efforts eventually paid off. He got stronger after every fight, and when he received various transformations, his power exponentially grew. It came to a point when he could defeat everybody he faced, including the strongest members of the Monster Association.
Even after everything he went through, Garo retained a fragment of his humanity. He only had a few phases of going berserk, but he always eventually regained his sanity. He also met Tareo during his journey, and the young boy became a reason for him to keep himself in check. He never went too far when it came to fighting humans, even though his dream was to become the supreme evil. The moment he forgot to consider Tareo after acquiring immense power, the result was devastating. Saitama utterly defeated him, but it was already too late for the young boy.
Saitama's story isn't too different from Garo. He also started as an average human, but his path to unparalleled strength was training -- a very specific training routine, to be precise. Unlike Garo, power and strength easily came to Saitama. His power levels skyrocketed after consistent effort until he could obliterate anybody with a single punch. Similar to Garo, Saitama also met all his friends -- more importantly his disciple, Genos -- prior to reaching his peak, and they were also the main reason he kept himself grounded and human.
The only time Saitama emitted murderous intent was after witnessing Genos' death. He went all out against Garo without any regard for the planet's safety. Though he returned to his nonchalant self after widening the gap in power between him and the Hero Hunter, it was evidence that Saitama himself wasn't immune to losing himself, and it was well-represented by the fact he held Genos' core even while dishing out planet-destroying techniques. The moment he let go of it may be the moment he also lets go of his humanity. Once that happens, Saitama can become the embodiment of apathy, not caring what happens or what havoc he can possibly cause in the world of One-Punch Man. Needless to say, that may also be the day the entire world will meet its doom.