The Biggest Mistake Voldemort Ever Made In Harry Potter
Over the course of the seven Harry Potter books (which are different from the films in a few key ways), the central conflict between Harry and Voldemort takes several unexpected, tragic turns. Harry suffers notable losses at the hands of Voldemort and his followers, and he spends much of the series in the dark — awaiting answers to the numerous mysteries scattered throughout the books. Fortunately, Harry does start to get the answers he's been looking for in the final two installments of the series: Harry Potter and the Half-Blood and Harry Potter and the Deathly Hallows. Many of them help explain the strange way he and Voldemort became so interconnected.
The many long-awaited solutions to the mysteries of the Harry Potter series don't just help to explain how Harry survived his numerous encounters with Voldemort, however; they also reveal the many mistakes that He Who Must Not Be Named made on his quest to kill Harry. There was one decision, in particular, that ultimately proved to be the biggest mistake Voldemort made in the Harry Potter series.
Voldemort should not have taken Harry's blood
At the end of Harry Potter and the Goblet of Fire, Voldemort creates a new physical body for himself to inhabit with the help of his loyal follower, Peter Pettigrew. Voldemort uses some of Harry's own blood to create his new body, and by doing so, manages to bypass the magical protection that had previously prevented him from making physical or magical contact with Harry.
However, the use of Harry's blood also has another side effect — one that Voldemort doesn't see coming, and it ends up costing him greatly. By taking Harry's blood, Voldemort actually anchors Harry to the physical world. This results in Voldemort being unable to kill Harry in Deathly Hallows, even when Harry willingly sacrifices himself. To make matters even worse for Voldemort, not only does his killing curse fail on Harry, but Harry's sacrifice results in all of his friends, classmates, and loved ones being protected from Voldemort's wrath — the same way his mother's sacrifice protected him. Harry (and the entire Wizarding World) essentially gets to reap the incredible magical benefits of a selfless sacrifice, without actually having to die — and it's all because Voldemort miscalculated.
If Voldemort had not taken Harry's blood, then he would have had a much better shot at succeeding in his quest to kill Harry and rule the Wizarding World. It was a choice he made out of arrogance, because he needed to see Harry die by his hand. He needed to prove that there wasn't anything special about that infant who rebuffed his curse all those years ago.
But, of course, there was something special about that infant, and Voldemort's ego ultimately cost him everything.