yeah they will just say "but others treat him like shit"
A lot of the time it’s because they love the fl that their personality changes. Some reasons are like “I have to grow stronger to protect her” or “I need to be more powerful so she won’t leave me.” It’s a lot to do with possessiveness and most of the time it happens because of jealousy, with the appearance of a second ml or something. They get scared of the fl leaving them, since they’re the only person that’s nice to them. So, instead of just believing the fl, they feel they need to do sneaky things or grow stronger to make SURE that she can’t leave them. It’s honestly kind of interesting, but most people just label it as yandere behavior lol.
Another reason sometimes is that in the middle of the story, the fl is forced to leave the ml and then they meet years later and the ml has changed from his perceived betrayal. However, he realizes he still loves her and then they end up together after some chapters of angst lol
So in the original novel he becomes a villain, but when the FL meets him he's a sweet and clingy child. FL raised him with lots of care and love, so how come he became an edgelord anyway? It's kinda stupid.