H eavy hitters
For all his bluster about the purity of blood, for all he believes his own propaganda most of the time, it is Mad-Eye Moody, halfblood and half-man according to some, after whom Voldemort goes first on the night they severed the protection charm and seven Potters moved in the night sky.
Mad-Eye was the biggest threat, just as Harry had been the bigger threat than the pureblooded Neville. Just as Albus Dumbledore was the only man he ever feared.
He is a halfblood himself, so he knows the power – if you can harness it – of two conflicting passions, two conflicting selves, warring in one body. It can drive you mad, and it can make you do anything – things neither Mudblood or pureblood has ever imagined. Mad-Eye Moody is a halfblood and the most dangerous foe in the night skies, so he is the one that Voldemort is certain to kill.
Purebloods fall into line behind a leader, it is their instinct after following their Head of Family from the beginning. Mudbloods cower, shrinking away in confusion and pain and regret, or stand to be devoured by dying proudly, never amounting to much. Halfbloods, on the other hand, have the right to both worlds, and they refuse to follow or to bow to those who have a birthright only to one.
They are volatile and dangerous and even when they are on your side you never know if they can be trusted. Halfbloods are different. They could do anything. They are the ones to eliminate, so that the world is black and white with no gray. Black will destroy white in a world of extremes, but gray is the most powerful shade of all.