The following contains spoilers for One-Punch Man Chapter 159, "Source of Anxiety," by One, Yusuke Murata, John Werry and James Gaubatz, available in English at Viz Media.
One-Punch Man isn't lacking in characters with interesting power, fighting style or personality. However, not many of them offer depth. Most are simply there for the sake of entertainment, either as a gag or some form of mockery. Garo is one of the few who actually has a story to tell with a respectable background arc -- though his motivations are questionable.
When Garo made his debut, he announced he was siding with the monsters. He claimed to sympathize with the villains and vowed to destroy the heroes. However, his power was still lacking then so he sought to fight stronger opponents to hone himself and his skills. He challenged S-Class heroes and even hospitalized some of them, but he also defeated countless strong monsters and had wholesome moments along the way. One-Punch Man Chapter 159 finally clarifies whether Garo stands on the side of good or evil.
After Centisennin and Evil Ocean Water were dealt with, Saitama went toward Garo and asked the Hero Hunter what exactly he is. But before he arrived, Garo recalled that the S-Class heroes were knocked out and yet to reappear. He assumed they were out of commission, supposedly meaning he no longer had anything to fear. Garo could finally perfect absolute evil using the martial art form he created. Despite this, he instead felt that there was something he was missing -- then Saitama arrived.
Garo did not recognize Saitama despite having met the bald hero a couple of times previously in One-Punch Man. He took a moment to think about Saitama's question, casting away his master's advice to follow his heart. Resolving himself to accomplish the task that he believed only he could do, he introduced himself as the monster Garo, then followed by claiming to cast all of humanity into the depths of despair and become the absolute evil.
Garo's decision isn't surprising, as he's always been dead set on pursuing the evil path. In fact, it's his primary motivation to get stronger. It's just that One-Punch Man has made it seem that Garo was finally turning over a new leaf. He painstakingly protected the kid Tareo on multiple occasions and even joined hands with Metal Bat -- someone he had a bad history with -- in order to defeat Centisennin. Garo also had some sort of enlightenment after going head-to-head against his former master, which allowed him to regain his humanity.
With Garo cementing his goal to be the ultimate evil, it's probably safe to assume that the only means to make him change his mind is to utterly destroy him. Fortunately, the most capable individual for that job is already on the scene.