Voldemort torments Harry Potter through legilimency in Harry Potter and the Order of the Phoenix, but his trick to goad Harry into killing Bellatrix Lestrange makes no sense. After the tragic death of Sirius Black, Harry pursues his murderer, Bellatrix Lestrange, and nearly kills her, but hesitates, prompting Voldemort, who’d been invading Harry’s mind throughout the year, to goad him into killing her. The moment appears to misunderstand the nature of the Harry Potter franchise’s Dark Magic, and if Harry had listened to Voldemort, the Dark Wizard’s cult of Death Eaters would have lost one of its most powerful members.
Among the most dangerous forms of Dark Magic in the Harry Potter franchise are the three Unforgivable Curses, which were capable of instantly killing, brutally torturing, or completely controlling the minds of others, respectively. The three curses are frequently used by Voldemort and his followers, the Death Eaters, with his most trusted subordinate, Bellatrix Lestrange, notably torturing Neville Longbottom’s parents before the events of the Harry Potter books and films and using the killing curse to murder Sirius Black in the film version of Order of the Phoenix (while the book instead had her incapacitate him with a non-lethal spell, causing him to fall into the Veil to his death).
To avenge Sirius, Harry stuns Bellatrix with a failed torture curse, which only manages to knock her to the floor, but it’s here that Voldemort tries to convince Harry to use the killing curse on Bellatrix, though these attempts are soon thwarted by the arrival of the powerful wizard Albus Dumbledore. Voldemort’s attempts to trick Harry into killing Bellatrix would have done far more harm than good to his sinister cause, as Harry killing Bellatrix might have resulted in dire consequences for him, but he’d remain one of Voldemort’s greatest threats and the Dark Wizard would be missing his most trusted lieutenant. Dark Magic is not shown to corrupt its users quite as the scene suggests, and Voldemort is shown in other material to care for Bellatrix, making the scene nonsensical in an otherwise dramatic sequence.
Harry Potter Killing Bellatrix Wouldn't Have Helped Voldemort
Unlike the dark side of the Force in the Star Wars franchise, the Wizarding World’s Dark Magic does not have addictive properties, so one use of the killing curse on a ruthless murderer would not suddenly turn Harry Potter into a dark wizard. Harry and his allies use unforgivable curses throughout the Second Wizarding War without such consequences. Even if Harry did become corrupted by dark magic, there’s no evidence that he’d be loyal to Voldemort and the Death Eaters. What might happen, however, is Harry being sent to Azkaban for successfully casing an unforgivable curse at a time when the Ministry of Magic outlawed their use with few exceptions.
This scenario wouldn’t help Voldemort’s cause as much as it hurt it, however, as Harry Potter would still be alive and the Death Eaters would have lost one of their greatest members in an already costly battle. Moreover, Voldemort is shown to care about Helena Bonham Carter's Bellatrix Lestrange on some form of an emotional level, as shown in the book version of Harry Potter and the Deathly Hallows, where he nearly kills Molly Weasley to avenge Bellatrix. Harry Potter and the Order of the Phoenix misunderstood both Dark Magic and Voldemort’s relationship with Bellatrix Lestrange by having him nonsensically try to trick Harry Potter into killing her.