That's fair, but Mangoku was also driven by a deep hatred of the Qin. As ridiculous as Hara was in trying to redeem him even a little bit, you understood the reasoning of his violence. Mangoku was made.
Han Roki wasn't moved to cruelty by anything other than want, whim and opportunity. He delighted in the slaughter of innocents so much, he quite literally left a trail of dead bodies that became his undoing because it led the HSU right to him. Han Roki was basically the kid that enjoyed pulling wings of flies given an army and free reign over defenseless civilians.
Ultimately, Kingdom isn't lacking for cruel, fucked up characters so I'm not disagreeing with your take, just standing by mine.