Why would telling the truth about the night in the schoolyard be similar to killing a mockingbird?

Boo did NOT kill Bob Ewell. Atticus tells his kids that they have no reason to go around harming mockingbirds since they have done no harm to them.

Explanation:

Sheriff Tate decided to cover-up the true nature of Bob's death. By declaring Boo Bob's killer--though his actions were in self-defense and Boo would almost certainly have been exonerated--it would have brought Boo into the "limelight" of a public investigation. Boo had carefully hidden himself from public view for decades, and Tate knew that a public trial would be the worst thing possible for Boo--a hero who had saved the children's lives.