Answer:
”It's a sin to kill a mockingbird" the mockingbird is a symbol of innocence, and he is saying that you should not harm the innocent, and the mockingbird is not a threat to anyone just like someone who is innocent/pure at heart is not a threat so if we harm them we are doing wrong and therefor it would be a sin to do such.
in proverbs 1 it talks about hurting the innocent, i think that this would be something that would work well for your question, because Solomon the man who wrote most of the proverbs was a father Figure just like atticus and here they are talking about the same thing
Proverbs 1:8
My son, hear the instruction of thy father, and forsake not the law of thy mother:
Proverbs 1:9
For they [shall be] an ornament of grace unto thy head, and chains about thy neck.
Proverbs 1:10
My son, if sinners entice thee, consent thou not.
Proverbs 1:11
If they say, Come with us, let us lay wait for blood, let us lurk privily for the innocent without cause:
Proverbs 1:12
Let us swallow them up alive as the grave; and whole, as those that go down into the pit:
Proverbs 1:13
We shall find all precious substance, we shall fill our houses with spoil:
Proverbs 1:14
Cast in thy lot among us; let us all have one purse:
Proverbs 1:15
My son, walk not thou in the way with them; refrain thy foot from their path:
Proverbs 1:16
For their feet run to evil, and make haste to shed blood.
Proverbs 1:17
Surely in vain the net is spread in the sight of any bird.
Proverbs 1:18
And they lay wait for their [own] blood; they lurk privily for their [own] lives.
Proverbs 1:19
So [are] the ways of every one that is greedy of gain; [which] taketh away the life of the owners thereof.
(if you wanna read the rest look it up, but this should help prove your point with what i wrote up there)