He wanted to rise and prove himself as a better magician in front of the others. Forging a team with the mysterious wizard was not an ideal thing for him. Later he realized that he has made a mistake because the wizard had some dark plans that he wanted to fulfill and now Alister was also a part of that plan.
Carol Monda has narrated something like this after a long time but for the author Cinda Williams Chima this was not the first time. The Demon King and Shadowcaster are also under the belt of the author and she happens to be an expert in writing fantasy tales. The story is a good one but it is the climax that will amaze you the most.