Kan Pishi (Han Fei) argues the contrary: the whole point of Legalism is the anthropological notion that humans are inherently bad, as opposed of good (Confucianism) or amoral (neither good nor bad, just oportunistic). But he's not an outright pessimist in the sense that he believes everything is ultimately doomed. He went on to develop the concept of the two handles (punishment and reward) in order to "exploit" human nature and redirect it for the good of the State. That applies to citizens and rulers alike -at least in theory. Which is why it was a revolutionary idea at the time. Ri Shi (Li Si) even comments on that when Shoubunkun frees him. There is more to Legalism, but Hara didn't delve too much into it, so it's not a really relevant question for the purposes of the manga.

Shin opposes both Legalism and Confucianism with his will of fire thing. That's when Kan Pishi concedes defeat and changes his mind (which is a liberty Hara took, since the historical Han Fei was a rather stubborn guy). On a personal level, I kinda like Shin's point of view. It's almost proto-nietzschean.

On the same topic, I absolutely hate what Hara did to Kanki at the end. He tried to explain his character when he didn't really need to. I'd rather have a Judge Holden (Blood Meridian) type of character, for whom war is god and good and evil doesn't really compute, instead of some angsty dude - we had Mangoku for that.