:0! YO that’ll be a very interesting twist !!! I wannna see that ヾ(☆▽☆)
I can imagine him becoming one after he prevents the rest of the students from becoming evil mages then all the students will have to work together to stop him ლ(´ڡ`ლ)
He is already an evil mage because he kills people for a living. The difference between him and the students in the future is that they use their powers to destroy the world (or kingdom, I can't remember).
I think evil refers to their magic getting out of control rather than how they use it.
In the first chapter he literally says "As soon as you start using your magic to kill people you become an evil mage, and I am no exception."
It feels like Solo-sensei might become an evil mage