I don't care about my ex's feelings
Why do you not care about your ex's feelings?
It's amusing to watch her get treated badly by her new boyfriend.
You are not required to genuinely care about an ex on any level. That said, it's truly mean to not care about their feelings and be disrespectful, as the only thing that happens is that they are going to be hurt.
Yep, I don't give a butthole about any of that.
That's a bad attitude to have. You won't be able to move on if you love seeing your ex hurt which only ends up hurting you.