In the past, it had been suggested that L was "not all there."
Maybe it was true. The detective was well-known for his disregard for conventions, and sleep deprivation did nothing to help his ability to control his already faulty moral compass. He had two successors: one was called Near, and the other Mello. Near was his analytical, practical side, and Mello was his emotional, chaotic and destructive side. What he had just done was certainly along the "Mello" spectrum.
Bob had thrown cans at him. He would bruise. It would hurt. And, even if the building was starting to catch fire, there was something wonderfully freeing about seeing "silent" Bob wailing. Starting towards the door, L quickly broke the glass on the fire extinguisher, neatly with the little hammer, and tossed it to Bob. Resisting the urge to salute, he made his exit as noninflammatory as possible.