The student's first night back at Hogwarts for the 1996-97 school year only proved that. See, Severus was a naturally Dark-inclined wizard. Dark Magic just came easier to him. He was also slightly magic-sensitive; he could feel a witch or wizard's natural magical attunement. That ability helped when he helped his Snakes with their school work. Not all Slytherins were Dark. There were a handful of Light wizards in the Snake Pit, but more Grey and Dark.