@MonkeyMagic People do regret bullying others, especially when it leads to extreme suffering that you didn't intended to inflict. Women also tend to empathize with a victim more than men (most likely that is the reason why they try to hurt victims mentally ). Of course this is manga and every bad guy is straight out psychopath. But in real life, most likely those girls, would have some mental issues because of what they did to MC. Self destructive ones.
I talk from experience, I was friends with bullies. While men forgot about it saying something like "Oh yeah, something like this happened", woman that bullied other started smoking and became alcoholic over time after finding out that their victim had scars on her wrists. (at least she thought that was what led to her issues)
Also studies also back that up look up any studies that involve empathetic behavior of woman compared to men (forgot real name but after some search you should find some interesting works).