Responses
He hasn’t changed personalities. Keep in mind that the MC ghosted him after witnessing what happened. However, this sunbae neither remembers cheating nor knows that the MC saw it. From his perspective, the MC’s decision to cut him off seemed unreasonable. Believing the ghosting stemmed from a simple misunderstanding, he lashed out in frustration. That’s why he brought up the MC’s lack of friends, something the MC had confided in him before—specifically, that his little brother was the cause. He acted out of frustration, and when you are frustrated, sometimes you want to hurt the person who hurt you.
Why did they changed sunbae personality? He would never say that, what's the point of making the rival likeable if you're going to change the personality... just make it am awful guy from the beginning