Umbridge was just the mean dean in Book 5, but in Book 7 she was unspeakably evil, staging witch hunts and siccing dementors on people to raise her status.
I even briefly considered writing a Harry Potter fan fiction where the villain is a wizard is avenging the soul of his Muggleborn wife who was killed by dementors and he goes around killing wizards who collaberated with the Death Eaters and avoided Azkaban, starting with Umbridge and gradually working is way towards less evil targets.
Also, he uses a gun in addition to his wand because most wizards don't know how to defend against it.
But really the only reason I wanted that story was for Umbridge to die.