When Atticus shot the mad dog, he inspired Jem and Scout (his kids) to become sharpshooters just like him. To give them early practice on their shooting skills he gets them a bb gun. When he gave them the bb gun he admonished them not to use the bb gun to kill mockingbirds. Mockingbirds were "sitting ducks" a common bird, usually easy to shoot and most people have a love-hate relation: they love the songs of the mockingbird, but hate the damage it does to glass (it attacks its own reflection). This relationship becomes a metaphor for the trial later on. Robinson is the mockingbird that Mr. Ewell is trying to kill. He should love Robinson (because Robinson loves his daughter), but he hates him because he's black. Instead of following Atticus' advice to his children, Ewell let's his hate drive him "To Kill a Mockingbird."
Thank you so much, tjbrewer! : )
That helps me understand perfectly, and now the whole storyline finally makes sense to me. : ) I plan to watch the movie again...