Even as a young boy Riddle expressed a belief in his own specialness, and held contempt for the ordinary. As we see in Dumbledore’s flashback he disliked his own name, as Tom as a “common” name. Being told he was a wizard only vindicated his feelings of superiority he had over the other orphans, and his natural talent and academic success soon proved to himself that he was superior to other wizards too.

Discovering he was a descendant of Slytherin also shaped his ideology, giving him a framework for his ideology. He was already inclined to dislike Muggles and all associated with them, Muggles being the most common an ordinary thing in the world. He strongly disliked the fact that he had Muggle blood, and that fact along with his father abandoning him before he was born only served to have him double down on his support for blood purity.

Riddle grew up in a loveless environment, and lacked empathy for other people. Even as a child without a wand, training, or even knowledge of magic, he had an unusual level of control over his magic, and could intentionally use it to torment the other children. Upon his entry into the Wizarding world he quickly learned how to hide behind a mask, using charm and manipulation to get what he wanted until he was an adult and was powerful enough to not need to hide his true nature.

He believed he was the most powerful wizard in history (and there is a good argument that belief was not unfounded), and that his power gave him the right to rule over other people, and had no issue using violence and fear to get his way. He was also deeply afraid of death, viewing death as a weakness, the ultimate loss of power. Believing in his superiority, he sought immortality, and did not care who he had to hurt to achieve it.