Vizet noticed Snape's expression. In his memory, it had been quite a long time since Snape had shown such a grave look.
The last time Snape had worn such a grave expression was probably several years ago, when Vizet had accidentally triggered the "empty shell" effect Voldemort had left behind, making Snape's arm ache.
Because of that incident, Snape had ended the original Potions private tutoring and changed it to private tutoring in the Patronus Charm.
Only after a series of events happened later did Vizet understand Snape's intention.
It turned out that Snape had thought he had been influenced by Voldemort, and since he had also mastered the "Soul Maze," Snape could only take a relatively roundabout method—proving he had not been influenced by Voldemort by having him summon a Patronus.
This time, however, the situation was different from last time. Compared with the grave expression mixed with vigilance from before, what showed more in Snape's grave expression now was worry.
Snape asked in an icy tone, "Have you, through some method or other, once again set foot on the Path of Magic that belongs to the Dark Lord?"
Hearing Snape say this, Vizet realized that what Snape was worried about now was actually somewhat similar to what had happened several years ago.
Snape was worried that the reason he could research and understand the Flying Charm was because, through some method, he had come into contact with Voldemort's Path of Magic, and then been influenced by it, which was how he had learned the Flying Charm.
"No..." Vizet shook his head. "Professor Snape, I 'glimpsed' his Path of Magic by understanding Voldemort's past."
Snape's expression remained grave. "Is that so?"
"Yes!" Vizet nodded. "In fact, as long as someone is a normal person, it's very hard for them