Upon rewatching Doctor Strange, the rationale provided by Mads Mikkelsen makes more sense.
Kaecilius is one of the only Marvel villains to actually kill someone important in an MCU movie, and it works because we understand his emotions in the process.
He feels lied to, and his turn to the dark side in many ways telegraphs the character arc of Chiwetel Ejiofor’s Mordo — which will receive much more attention and focus in future installments.
When the movie opens, Kaecilius simply seems like a violent psychopath.