One protagonist can be an antagonist for another protagonist. That also create conflict that can be used to drive plot forward and the resolution, good or bad, can create character growth. It can also be a way to make your protagonists more human, dealing with hurt, and loss, and resentment, and feeling lost and confused. I know that a mentee became antagonistic to their mentor in several stories, but as for the mentor becoming antagonistic, I feel like there are some where the mentor was evil all along, but not with the dynamic you're going for, so it should be interesting.