Question: Is "Dick" Tracy's first name, or his occupational designation?
Answer: His full name was Richard Tracy, but it was used seldomly.
Chosen answer: It's a bit ambiguous. Tracy's creator, Chester Gould, originally named his character, "Plainclothes Tracy," until an editor suggested changing it to Dick Tracy. A "dick" is a dated slang term for a cop or a private detective. Dick appears to be Tracy's first name, but it is also descriptive of his profession. This is called a "double entendre." It's typical for authors to contrive a name that reflects their characters.
Answer: He was babbling like an idiot, he was trying to justify his actions not only to Tess but himself. The "I love you so much I have to kill you" defense.