In chapter 10, atticus tells scout and jem that “it’s a sin to kill a mockingbird.” find three examples of that advice being echoed with similar imagery in chapter 25.   

When Scout goes to kill a roly-poly, Jem stops her. She asks, "“Why couldn’t I mash him?” and Jem replies, “Because they don’t bother you.” This is similar to the mockingbird quote because mockingbird's don't hurt anything, just like the roly-poly. They haven't earned their murder.
"He likened Tom’s death to the senseless slaughter of songbirds by hunters and children." This quote is much more straightforward in that it almost directly repeats the idea that killing a mockingbird is terrible. However, here it is specifically used to compare the mockingbird's death to Tom's. 
"Mr. Underwood simply figured it was a sin to kill cripples, be they standing, sitting, or escaping." This comes right before the second quote. It carries the same message that it's a sin to kill a mockingbird since a mockingbird can't defend itself, just like cripples can't be either.