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"'Remember it's a sin to kill a mockingbird.' That was the only time I ever heard Atticus say it was a sin to do something, and I asked Miss Maudie about it."
This quote is as applicable in our modern world as it was when the story was written. In fact, there is no time in history where the meaning behind Atticus' words doesn't fit in the picture.
Car bombings, murder, trafficking, gang initiations, abuse, terrorism..... all of these things include the death of innocents, people who do nothing more than increase joy in those around them.