Here's the text excerpt:
"They don't eat up people's gardens, don't nest in corncribs, they don't do one thing but sing their hearts out for us. That's why it's a sin to kill a mocking bird."
1.) Mockingbirds don't take. They do nothing to harm us.
2.) They don't get in our way. They stay to themselves.
3.) They give us their singing to enjoy.
To kill a creature that only gives you beautiful singing, that takes nothing from you, that's a sin.