Why does Atticus defend tom Robinson even though Atticus is aware of the roadblocks he will encounter and the possible negative effect upon his children?

Read the book To Kill a Mockingbird very thoroughly and watch the movie too. You will see Atticus realizes Tom Robinson was innocent and was being railroaded by bigoted people. Tom Robinson was NOT a piece of black trash like that community thought. It was the white guy who was the creep. Tom Robinson also had an infirmity. Read the book.