Despite the death and destruction that Voldemort caused during his lifetime, viewers across the globe were happier to see Umbridge go first. Umbridge ignites the fury in your soul that takes over your mind and body whilst the red mist descends, making anger feel like fire. But let's just take a moment to remember that she's not real... Of course, people hate her because there are real people like her in society that almost everyone has encountered before. They use their power to dictate over others and they will make your life hell if your opinions differ to theirs. In this case, Professor Umbridge believes that Harry is lying about Voldemort's return and forces him to write lines using his own blood. Harry is permanently scarred because of Umbridge's medieval torture punishments so when that is combined that with her lifeless eyes and decaying soul, Rowling has created a villain who is hated more than Voldemort. Then she rationally comes to the conclusion that the only way to find out the truth is to use the Cruciatus Curse on Harry after she's used all of the Veritaserum on Cho Chang. Makes sense.
