Fear generally makes people stupid in the Harry Potter novels. A great example of this is Cornelius Fudge, the Minister of Magic. In Goblet of Fire, he's so committed to his denial of Voldemort's rise that he refuses point blank to believe Harry's eyewitness testimony. What's more, he rejects all of Dumbledore's excellent advice about not letting the Dementors maintain control of Azkaban Prison (this right after watching a Dementor eat Barty Crouch, Jr.'s soul before they can get an official statement from the guy). The Death Eaters thrive on fear because it causes social division and confusion among their victims. It also makes people like Cornelius Fudge easier for the bad guys to control.