At least in his own mind, Lord Voldemort is a superior being. He's magical powers are close to unprecedented, he has genius level intellect (and had it from an early age as a brilliant student) and he's generally the walking epitome of his Magic Is Might manifesto. Except, he's dead and he was defeated by a group of school-children, essentially. So what gives?
It turns out that Voldemort's defeat wasn't merely down to his enemies' prowess - he basically helped gift wrap his demise for them with some of the most ludicrous, ill-judged decisions any great villain has ever made. It seems that in the interest of hobbling him enough to make him appear vulnerable to defeat, JK Rowling decided the best way was to make him part Near-Invincible Master of the Dark Arts and part dribbling moron...
10. Punishing Malfoy By Undermining His Own Plans
For all of his concerns about wizarding supremacy, Voldemort doesn't half stretch the logic when it comes to hiring his Death Eaters. Other than Barty Crouch Jr, they're mostly just idiots - loyal, but moronic - though, Lucius Malfoy does stand apart somewhat. Unfortunately for him, though, his failures are seemingly that little bit more intolerable to the Dark Lord, so he finds himself ostracised and alienated.
And if that wasn't enough, Voldemort hand-picked Draco Malfoy to do his dirty bidding and create a way for the Death Eaters to enter Hogwarts. Oh and also killing Dumbledore. It's not just about having an inside man, though - Draco is chosen because of what it means to the Malfoys. If he fails, he will end up punished and if he succeeds, the murder will break him. This is Voldemort's reward to them for Lucius' failings.
It's stupid, though, because it puts a vulnerable, inadequate wizard in a supremely important position and it turns the Malfoys against Voldemort so much that Narcissa lies about Harry being dead in the final battle, betraying her Lord to his defeat.