Why doesn't the book "To Kill A Mocking Bird" have nothing to do with killing Mocking Birds?

It did teach me a valuable lesson about not judging people on the colour of their skin but it was no use to me as it didn't teach me how to kill a mocking bird, so the question is
Why doesn't the book "To Kill A Mocking Bird" have nothing to do with killing Mocking Birds
Update: Yes I actually did read the book and learn it chapter by chapter, learnt every main character traits and every metaphorical image and saying, for an exam. I also saw the film
Update 2: Rephrasing of question, why does the book to kill a mocking bird have anything to do with killing mocking birds
Update 3: Rephrase again, Why does the book to kill a mocking bird have nothing to do with killing mocking birds
Update 4: Does Dill really grow up to be Dirty Harry as in the Clint Eastwood films
Update 5: I think it's unfair Atticus shoots a harmless rabid dog dead
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