You have to see this from HER pov
According to her, Harry Potter was lying, according to her, Voldemort was dead, according to her, the ministry was more powerful than Dumbledore. According to her, Dumbledore had a secret society against the ministry. She is a Slytherin. (Slytherins value cunningness, intelligence, ambition, power)
As the undersecretary, she hardly got to make her own dessicions it was the minister who gave the instructions. When he appointed her as the teacher, I bet she gladly accepted because, she finally got the attention she always wanted, (who cares about the children?) She could do whatever she wanted as fudge won't fire her, as he was the one who appointed her. (Ah that satisfaction she got when she could inspect the teachers! that superiority!)