The Janitor was one of two Forerunners that possessed some degree of intelligence, the only other being The Librarian. He was responsible for the cleanup of all the Halo Installations. Obviously since this task was impossible, he created an entirely new race of robots that he called the Sentinels. These robots were designed to clean up the Halo rings using their beams that were filled with flaming Cillit Bang, that was designed to burn away dirt.
When the Forerunner council saw these robots however, they assumed that they were designed to help fight the Flood, because they're all idiots. Despite the Janitors protests, the Sentinels replaced the superior Prometheans on every single Halo Installation, and even their corresponding Gas Mines.
The Sentinels proved majorly ineffective against the Flood, however apparently Cillit Bang that has been set alight is quite good against shields...
The Janitor then spent the rest of his days on the Raston science facility, attempting to design a warrior robot to make up for his failure. Unfortunatly for him, it went rogue and killed him.