What was meant by the quote "I don't know, but they did it... Good night"

Atticus says this in response to Jem’s question, “How could they do it, how could they?” which, the text says, Jem asks “bleakly” because he is very upset about the jury’s decision to find Tom guilty in the face of all evidence to the contrary.  Atticus means that injustice is common, that many times people will make the easy decision instead of the right decision, and that only children have the sensitivity to understand this enormous gap between right and wrong, and because they understand it, be sad and lament such human weakness.