First of all, you cannot compare Mean Girls to Cruel Intentions. Mean Girls is a PG-13 movie, while Cruel Intentions is a hard R. Secondly, Kathryn wasn't mean. She was pure evil. She is an emotionally unstable psychopath who likes to use other people as pawns for her entertainment. She uses them as toothpicks and throws them away, destroying lives as she goes along. She did drugs, had sex with men, and fooled everyone around her into thinking that she was an inspirational angel, but few know her true, dark personality. Kathryn could literally kill Regina, the same way that she indirectly murdered her brother and didn't show any sign of sadness or regret. Regina was a simple wannabe in high school who thinks she's all that, but there's someone prettier, richer, and more popular than her.