The Case of the Courteous Killer

The first two bodies were found in a lover’s lane... His girl friend, still alive, described the stick-up artist as mild-mannered, courteous, of medium build and wearing glasses. It was a description that fitted half the male population of Los Angeles. It was almost the only clue Joe Friday and Frank Smith had to catch the murderer.
The courteous killer struck again! Then again! This time Joe Friday was waiting for him with a gun. But the criminal escaped with only a bullet wound.
When Joe Friday opened his mail, there was an unsigned, misspelled note. It read: “You chink your a smart badge. Nobody burns me and lives, cop. Start swetting.”