Answer:
In saying 'Bob Ewell fell on his knife' Heck Tate was trying to protect Boo Radley.
When Scout says “Well, it’d be sort of like shootin’ a mockingbird wouldn’t it?” she was referring to the time Atticus told her it is forbidden to kill a mockingbird because it is always just trying to help everybody. She means that Boo Radley is the mockingbird here who was only trying to help them, and exposing what he did will result in the people pestering him which would injure his shy and private nature.
Scout finally understood what Heck Tate was trying to tell her father, that Jem did not kill Bob Ewell and that he is trying to protect Boo from being flung into the public eyes by saying Bob fell on his knife.
Explanation:
After Bob Ewell was killed Atticus and Heck Tate discussed the situation. Atticus thought Jem killed Bob and should be brought to justice, Heck Tate insisted that Jem did not kill Bob and that Bob "fell on his knife".
Heck's insistence on this is because he knows Boo Radley is the actual killer, but Boo was only stopping a crime. He thinks its best if the town people didn't know that Boo saved the children, as they will start treating him like a hero and cost him great pains due to his shy nature.
Scout understood what he meant finally and told her father Heck Tate is right, because it will be "like shooting a mockingbird", meaning exposing what Boo did will cost him his much treasured privacy when he was just trying to help.
