O Lorde das Trevas não sabia que Harry tinha um pedaço de sua alma.

Ao contrário das Horcruxes que ele criou, o Lorde das Trevas não tinha intenção de colocar um pedaço de sua alma em Harry Potter. Quando ele tentou matar Harry e sua Maldição da Morte se recuperou, isso fez com que um pedaço de sua alma se separasse sem sua intenção ou conhecimento, e se ligou a Harry. Sua alma já estava enfraquecida devido a ele tê-lo dividido sua alma tantas vezes para fazer as Horcruxes que ele tinha intencionalmente criado, então quando sua maldição se recuperou, isso fez com que sua alma se dividisse.

“You were the seventh Horcrux, Harry, the Horcrux he never meant to make. He had rendered his soul so unstable that it broke apart when he committed those acts of unspeakable evil, the murder of your parents, the attempted killing of a child. But what escaped from that room was even less than he knew. He left more than his body behind. He left part of himself latched to you, the would-be victim who had survived.” _{^{- Harry Potter and the Deathly Hallows, Chapter 35 (The Flaw in the Plan)}}

Quanto mais Horcruxes um mago faz, mais instável fica sua alma. O Lorde das Trevas criou cinco antes de sua tentativa de matar Harry, o que fez com que sua alma ficasse extremamente instável.

Voldemort não sabia.

"Here is the thing: for convenience, I had Dumbledore say to Harry, "You were the Horcrux he never meant to make," but I think, by definition, a Horcrux has to be made intentionally. So because Voldemort never went through the grotesque process that I imagine creates a Horcrux with Harry, (SU: Mm-hm.) it was just that he had destabilized his soul so much that it split when he was hit by the backfiring curse. And so this part of it flies off, and attaches to the only living thing in the room. A part of it flees in the very-close-to-death limbo state that Voldemort then goes on and exists in. I suppose it's very close to being a Horcrux, but Harry did not become an evil object. He didn't have curses upon him that the other Horcruxes had. He himself was not contaminated by carrying this bit of parasitic soul." — Pottercast 12.23.07 - The Leaky Cauldron

Harry não foi deliberadamente transformado em horcrux, então Voldemort não tinha motivos para evitar matá-lo.

Voldemort não sabia que ele havia criado uma horcrux fora de Harry - depois de fazer tantos antes de Harry, ele tornou sua alma muito frágil, repetidamente rasgando-a em pedaços para escondê-la. Quando a proteção de Lily sobre Harry impediu Voldemort de matá-lo, a alma gravemente danificada de Voldemort se rompeu novamente, e o fragmento que se rompeu se escondeu dentro da própria alma de Harry, como ele era a única coisa viva na casa, e Voldemort não tinha feito o específico ritual para transformar um objeto em um lar adequado para uma horcrux. Voldemort não estava ciente do dano, porque muito mais estava acontecendo - ele perdeu o corpo e estava fraco ("mais fraco que o pior fantasma").

"'You can speak Parseltongue, Harry,' said Dumbledore calmly, 'because Lord Voldemort--who is the last remaining ancestor of Salazar Slytherin--can speak Parseltongue. Unless I'm much mistaken, he transferred some of his own powers to you the night he gave you that scar. Not something he intended to do, I'm sure...'"
_{Chamber of Secrets pg 245}

"'... Tell him that on the night Lord Voldemort tried to kill him, when Lily cast her own life between them as a shield, the Killing Curse rebounded upon Lord Voldemort, and a fragment of Voldemort's soul was blasted apart from the whole, and latched itself on to the only living soul left in that collapsing building. Part of Lord Voldemort lives inside Harry, and it is that which gives him the power of speech with snakes, and a connection with Lord Voldemort's mind that he has never understood. And while that fragment of soul, unmissed by Voldemort, remains attached to, and protected by Harry, Lord Voldemort cannot die.'"
_{Deathly Hallows pg 551}

"'You were the seventh Horcrux, Harry, the Horcrux he never meant to make. He had rendered his soul so unstable that it broke apart when he committed those acts of unspeakable evil, the murder of your parents, the attempted killing of a child. But what escaped from that room was even less than he knew. he left more than his body behind. He left part of himself latched to you, the would-be victim who had survived."
_{Deathly Hallows page 568}

Nota: estes são os números de página do conjunto de hardcovers de 2007 da capa dura de Harry Potter, eles podem variar ligeiramente de edição para edição.