In my opinion Mads Micklesons character: Keacilius is vastly under rated. He has motivations, pathos, and is a dark reflection of Steven Strange, physically and psychologically. Allow me to explain.
The film Dr. Strange is really about time, life, and control. While Strange begins the film with a collection of expensive watches, by the end the only one he owns is a broken keepsake with only sentimental value. This reflects his change from a man who is cynical and in control, to a more sentimental person, who through his adventures in the movie understands he cannot control time.
The character of Keacilius is a character who something bad happened to in the past, which led to his turn to the dark side. It is hinted his entire family may have died. This causes him to search for control. He joined the sorcerers to learn how to prevent death. That is his goal throughout the movie. Preventing death, and creating unending life. He is taught Dormammu is evil but when he learns the ancient one draws her power to live forever from him, Keacilius feels betrayed and goes rogue. He believes sorcerers keep world changing power for themselves, instead of allowing it into the world to save everyone, and prevent death. In the end though, the life without end promised by Dornammu is revealed to be a fate worse than death. Keacilius and the Zealots are transformed into Mindless Ones. These are servents of Dormammu who lack minds, souls, and individuality. Kaecilius gains his wish for immortality, but it is at the cost of humanity, individuality, and everything that makes him human. He would be better off dead, but he can't die.