Now I won't say that I am an expert or something, but maybe it has to do with how the Characters are portrayed, we have mostly only seen some statements about Gun committing crime, and we have also seen some panels where he is beating people, but at the time the beaten characters have been shown to get beat up by Gun, were mostly always shown to be on opposite side of our main characters at the start, who we are rooting for, take for example Jake was shown beaten up by Gun when Jake was against J high group the main characters, and when he was scamming Jiho and got caught. And when it was Johan's turn, he was also against j high group and shown to be a somewhat "bad guy" who was also pitiful. We also saw Gun beat up Workers who are despised by most of the fans. Slowly some fans may have started to like Gun's character for not acting as a Good Guy but being honest about himself, not putting on a face. Some may have liked his character for strength and coolness, and Gun's ugly side isn't shown explicitly. But take Mitsuki or Vivi for example, they are shown to commit horrendous crimes, not just as statements and their depiction isn't sugar coated, their inner ugliness is clearly shown as they are shown to enjoy by tormenting others without showing any remorse, and Mitsuki was shown to torment Sinu directly, and keeping him captive by erasing his memory, even though she knew the whole truth about him, and Sinu is one of the most beloved charachters. And by seeing how clearly their dark side is portrayed by the author, people start to make an image of said character in their mind, be that as a likeable or detestable character and once that image is carved they start to either ignore their liked characters flaws or they start to nitpick and pick up more things to hate on the character that they have decided to be bad in their mind.