Something that I didn’t mention is that Mimi basically accidentally killed 14 patients by accidentally mixing up the medicine because her boss (Dr.Grey) overworked her way too much, causing her to mess up, and then began to resent him
So basically after accidentally killing 14 patients which she caused and was part Dr. Grey’s fault and accidentally killing her beloved sister in a car crash, she began to hate Dr. Grey, and herself, and then decided to become her sister to throw away herself and kill her boss