They believe she’s withholding secrets that should not be withheld, and think that maybe it’s not a bad thing if other dimensions absorb our reality.
They think that, in fact, could lead to benefits such as immortality.
It may also lead to destroying the entire world as we know it.
The antagonist is Mads Mikkelsen, who plays a character named Kaecilius who was a sorcerer within Kamar-Taj (where the Ancient One trains), who along with some other of his followers — who are called zealots — defect from Kamar-Taj, because they believe that the Ancient One is not being truthful in the way that she is teaching magic, is teaching sorcery.