Bartemius Crouch Jr.
Lord Voldemort employed countless cowards, psychos and sycophants in his rise to power, yet only the dark wizard's greatest servant makes this tragic list.
Shocked? You might well be. After all, Barty Crouch Jr. was no slouch. Not only did he infiltrate Hogwarts, he did so by kidnapping and impersonating the formidable Alastor "Mad-Eye" Moody. That took magical finesse, and everyone fell for it. Teachers and students alike took Crouch for the genuine article as he taught Defense Against the Dark Arts classes by day and secretly plotted to deliver Harry Potter to the dark lord during lunch breaks and planning periods.
Here's the kicker: Crouch succeeded! Yet so flawless was his execution, so perfect his disguise, that he actually proved an exceptional instructor to his young charges. He taught Potter and friends to overcome all three unforgivable curses and vanquish the forces of evil. Way to go, Crouch.
The lesson: A tireless work ethic is an excellent trait in a wizard, but maybe slack off a little when it comes to training your master's archenemy. Otherwise, what's the use in torturing innocents, killing your father and dying in the clutches of a Dementor?