Mockingbirds are absolutely innocent. There are other bird that may actually deserve to be killed for the pests that they become to humans. They steal seeds from fields and poop all over things. Mockingbirds, however, innocently sing and create pleasant sound for us.
The black man in this day and age did nothing but try to create pleasantries for the white man. They took their unfortunate place in society and tried to earn a decent wage by working for white men.
Atticus noticed that many white men cheated colored men because they knew the courts would find white men innocent even though they cheated black men. Courts were indeed affected by color. Atticus taught his children this principle and let it also ooze into his closing argument. It is not right to treat a colored man as any less of a man, Atticus thought. Atticus wanted to see innocence prevail no matter who the victim was. In this case, we are talking about Tom Robinson.