Voldemort, also known as Tom Riddle, is considered the Heir of Slytherin for several reasons.
First, Voldemort is a pure-blood wizard, descended from Salazar Slytherin himself through his mother’s ancestry. Pure-blood wizarding families, such as the Slytherins, value blood purity and often believe that they are superior to those with mixed or non-magical ancestry. Voldemort, therefore, fits the criteria of a pure-blood and is considered a suitable candidate for the role of Heir.
Second, Voldemort possesses a number of traits that are associated with Slytherin House at Hogwarts School of Witchcraft and Wizardry. Slytherin is known for its ambition, cunning, and resourcefulness, and Voldemort demonstrates all of these qualities in his quest for power and domination. He is also highly intelligent and manipulative, traits that are often associated with Slytherin students.
Third, Voldemort was sorted into Slytherin House when he attended Hogwarts, further solidifying his connection to the House and its values. It is not uncommon for children of pure-blood families to be sorted into Slytherin, as many of these families hold similar values and beliefs.
Fourth, Voldemort actively sought to align himself with the Slytherin legacy and used the symbols and symbols of the House in his branding and propaganda. He even named his followers the Death Eaters, a reference to the serpent, the animal associated with Slytherin.
Fifth, Voldemort sought to return to the ideals of Salazar Slytherin, who believed in the superiority of pure-blood wizards and was against the admission of Muggle-borns to Hogwarts. Voldemort, similarly, believed in the supremacy of pure-bloods and sought to rid the wizarding world of those he deemed inferior, including Muggle-borns and those of mixed ancestry.
Finally, Voldemort claimed the title of Heir of Slytherin for himself, further solidifying his connection to the House and its legacy. He saw himself as the rightful inheritor of Slytherin’s vision and sought to bring about a pure-blood dominated society in line with Slytherin’s beliefs.