The One Good Thing Barty Crouch Jr. Actually Did In Harry Potter
Lord Voldemort (Ralph Fiennes) is obviously the main villain of the "Harry Potter" series, but not every book and movie in the franchise features him as the main villain. In "Harry Potter and the Goblet of Fire," Barty Crouch Jr. (David Tennant) turns out to be the evildoer most directly responsible for forcibly entering Harry (Daniel Radcliffe) into the Triwizard Tournament and ultimately bringing a corporeal Voldemort back to the wizarding world.
"Goblet of Fire" marks a distinct turning point for the series, and Crouch Jr. proves by far the most dastardly Death Eater Harry meets up until that point in the series. The son of esteemed Ministry of Magic official Barty Crouch Sr. is among the Death Eaters directly responsible for brutally torturing Neville Longbottom's parents in the wake of Voldemort's disappearance, and he is subsequently imprisoned in Azkaban by his own father. Crouch Sr. eventually schemes to let his son out of Azkaban by replacing Crouch Jr. with his wife in the prison and keeping Crouch Jr. under control at home using the Imperius Curse. Crouch Jr. is formidable, though; he eventually slips the Imperius Curse and escapes, returning to Voldemort's service and taking on an undercover role by impersonating former Auror Alastor "Mad-Eye" Moody (Brendan Gleeson) and ensuring the events of that year's Triwizard Tournament lead directly to Voldemort's return.
While Crouch Jr. is firmly in the "evil" camp within the wizarding world, his actions in "Goblet of Fire" do have an unintended positive consequence for Harry and other Hogwarts students.
Barty Crouch Jr. is a quality Defense Against the Dark Arts teacher
Barty Crouch Jr.'s plan to help resurrect Voldemort involves a great deal of making sure Harry is prepared to survive the Triwizard Tournament. It seems that once Crouch Jr. goes through the trouble of impersonating Moody, he figures he might as well actually play the part and teach young wizards Defense Against the Dark Arts well while he's there. During "Harry Potter and the Goblet of Fire," Crouch Jr. teaches Hogwarts' fourth-year students quite a bit about the three Unforgivable Curses. While the lessons are intense and even involve Crouch Jr. practicing the Imperius curse on Harry and his friends, the experience helps Harry learn to fight off the same curse that Voldemort uses on him during the book's climax, helping him survive the book's climax. These lessons pay off in spades down the line; in the next story, Harry takes it upon himself to educate his friends on how to fight dark magic.
A recurring aspect of each "Harry Potter" book and movie is the revolving door of DADA teachers Harry, Ron (Rupert Grint), and Hermione (Emma Watson) meet throughout their years at Hogwarts. The professors always prove important to that book's story, although they vary in actual teaching ability. From the dreadfully incompetent teachings of Gilderoy Lockhart to the genuinely thoughtful and well-intentioned curriculum of Remus Lupin, all the way back to the malignant theoretical style imposed by Dolores Umbridge, Harry and his friends experience quite the lopsided education. Crouch Jr. (disguised as Moody) proves one of the better teachers the students have during his one year in office, despite the fact that he is literally a Death Eater.