At least in the novel, the MC admitted it was a stupid idea to use the Demon King technique in front of so many people in this scene. That's taken out of the manga. Because it was stupid. I get his friend was about to stab him in the back, but he'd already proven he could defeat his friend without theatrics. Even the Demon King tried to get him to make amends because the Demon King knew it was a stupid thing for the MC to do.
Everyone will take this as a reason to justify their hate for the parents and everyone else on top of other things. I'm not going to make an excuse for anyone, but I think this wasn't the best way to force the MC to run away from a writing perspective because people who were attacked by the Demon King are justified in their negative feelings toward him. I don't blame Sadith/Sadiz for her outburst. Of course the cliche trope of making the Demon King "misunderstood" will prevail for most people, but he literally wiped out an entire village/town/city for what, because one guy was messing around with magic in an underground lab? At least Treynar admits his faults and admits he was just as murderous as the human faction, but of course, because he's the MC's sidekick, the author will consistently lessen Treynar's past crimes and prejudices while highlighting current human crimes and prejudices.
Humans and Demons were enemies. That's like your country or family having a long heated rivalry with another country and family and go to war. Your country family uses a fighting style while the other country family uses a fighting style. You have a child and you raise them to use your fighting style and they don't excel at it and all of a sudden they beat up some of the strongest people in your country and family with your enemy's fighting style. You're going to have questions, your first emotional reaction is going to be a range of frustration, surprise, anger, disgust, annoyance, etc. until you collect yourself. If it's done in the midst of a crowd of people, they're going to have justified reactions similar to yours, but yours will likely be a stronger reaction since that's your child. Unless you absolutely don't care about your child which isn't the case for MC's parents despite their negligence.
Like I said, I wish the mangaka left the admission in there by the MC that using the Demon King special technique was stupid.
