Geralmente os magos não gostavam de fazer as coisas do jeito trouxa. Voldemort era um supremacista dos feiticeiros e odiava os trouxas. Ele até mudou seu nome porque recebeu o nome de um trouxa.
“It was a name I was already using at Hogwarts, to my most intimate friends only, of course. You think I was going to use my filthy Muggle father’s name forever? I, in whose veins runs the blood of Salazar Slytherin himself, through my mother’s side? I, keep the name of a foul, common Muggle, who abandoned me even before I was born, just because he found out his wife was a witch? No, Harry — I fashioned myself a new name, a name I knew wizards everywhere would one day fear to speak, when I had become the greatest sorcerer in the world!”
Harry Potter and the Chamber of Secrets, Chapter 17, The Heir of Slytherin
Voldemort sempre quis ser diferente e superior aos outros. Ele não pode ser superior se ele usar faca ou arma.
“I knew I was different,” he whispered to his own quivering fingers. “I knew I was special. Always, I knew there was something.”
“Well, you were quite right,” said Dumbledore, who was no longer smiling, but watching Riddle intently. “You are a wizard.”
Harry Potter and the Half-Blood Prince, Chapter 13, The Secret Riddle
Ele poderia querer usar Harry para fazer sua sexta Horcrux.
“Well, it is inadvisable to do so,” said Dumbledore, “because to confide a part of your soul to something that can think and move for itself is obviously a very risky business. However, if my calculations are correct, Voldemort was still at least one Horcrux short of his goal of six when he entered your parents’ house with the intention of killing you. He seems to have reserved the process of making Horcruxes for particularly significant deaths. You would certainly have been that. He believed that in killing you, he was destroying the danger the prophecy had outlined. He believed he was making himself invincible. I am sure that he was intending to make his final Horcrux with your death. As we know, he failed. After an interval of some years, however, he used Nagini to kill an old Muggle man, and it might then have occurred to him to turn her into his last Horcrux. She underlines the Slytherin connection, which enhances Lord Voldemorts mystique; I think he is perhaps as fond of her as he can be of anything; he certainly likes to keep her close, and he seems to have an unusual amount of control over her, even for a Parselmouth.”
Harry Potter and the Half-Blood Prince, Chapter 23, Horcruxes
Ele teria achado desagradável usar o modo comum de matar trouxa. A resposta simples para a sua pergunta é que o ego dele não permitiria que ele usasse formas trouxas.
But I knew, too, where Voldemort was weak. And so I made my decision. You would be protected by an ancient magic of which he knows, which he despises, and which he has always, therefore, underestimated - to his cost. I am speaking, of course, of the fact that your mother died to save you.
Harry Potter and the Order of the Phoenix, Chapter 36, The Only One He Ever Feared
E o mais importante, ele não esperava que o feitiço fosse contra ele. Voldemort matou muitas pessoas e não enfrentou problema semelhante. Então, era natural que ele usasse uma maldição.