A Fear of Fire
Although I believe Saruhiko's reasons for leaving Homra primarily come from his desire to leave Misaki before Misaki abandoned him and the better offer from Munakata Reisi it is important to note that they are not the only reasons why he left. A third reason, a fear of fire also contributed to his departure.
Saruhiko's fear isn't crippling. In fact it he usually manages it well. He is able to fight against most of Homra without incident and he can wield the flames of the red clan without fear but joining Homra made him relive the worst of his childhood fears. He only outwardly shows his fear when he is close to Suoh Mikoto's flames. This fear allows him to keep his head and not confront him when there's nothing he'd like more.
Saruhiko's father, Fushimi Niki in one of his worst abuses of his son burned his school project and threw cockroaches on it. The entire incident understandably left Saruhiko traumatized and joining Homra made him remember just how much that hurt. He felt as if he was burning from the inside out and even after it stopped he was never able to completely forget it. Wielding that power was a reminder of what his father did and perhaps made him think he might be becoming like his father in a way. He finds the blue aura calming by comparison and it acts as a balm against the flames that burn inside him.